diff --git a/src/Umbraco.Core/Migrations/IMigration.cs b/src/Umbraco.Core/Migrations/IMigration.cs
index 53b7874b3a..c929234f77 100644
--- a/src/Umbraco.Core/Migrations/IMigration.cs
+++ b/src/Umbraco.Core/Migrations/IMigration.cs
@@ -7,6 +7,9 @@ namespace Umbraco.Core.Migrations
     /// </summary>
     public interface IMigration : IDiscoverable
     {
+        /// <summary>
+        /// Executes the migration.
+        /// </summary>
         void Migrate();
     }
 }
diff --git a/src/Umbraco.Core/Migrations/IMigrationContext.cs b/src/Umbraco.Core/Migrations/IMigrationContext.cs
index 4db1b07b63..80ba78b6de 100644
--- a/src/Umbraco.Core/Migrations/IMigrationContext.cs
+++ b/src/Umbraco.Core/Migrations/IMigrationContext.cs
@@ -24,8 +24,13 @@ namespace Umbraco.Core.Migrations
         ISqlContext SqlContext { get; }
 
         /// <summary>
-        /// Gets the expression index.
+        /// Gets or sets the expression index.
         /// </summary>
         int Index { get; set; }
+
+        /// <summary>
+        /// Gets or sets a value indicating whether an expression is being built.
+        /// </summary>
+        bool BuildingExpression { get; set; }
     }
 }
diff --git a/src/Umbraco.Core/Migrations/IncompleteMigrationExpressionException.cs b/src/Umbraco.Core/Migrations/IncompleteMigrationExpressionException.cs
new file mode 100644
index 0000000000..91d1838d6f
--- /dev/null
+++ b/src/Umbraco.Core/Migrations/IncompleteMigrationExpressionException.cs
@@ -0,0 +1,28 @@
+using System;
+
+namespace Umbraco.Core.Migrations
+{
+    /// <summary>
+    /// Represents errors that occurs when a migration exception is not executed.
+    /// </summary>
+    /// <remarks>
+    /// <para>Migration expression such as Alter.Table(...).Do() *must* end with Do() else they are
+    /// not executed. When a non-executed expression is detected, an IncompleteMigrationExpressionException
+    /// is thrown.</para>
+    /// </remarks>
+    public class IncompleteMigrationExpressionException : Exception
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="IncompleteMigrationExpressionException"/> class.
+        /// </summary>
+        public IncompleteMigrationExpressionException()
+        { }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="IncompleteMigrationExpressionException"/> class with a message.
+        /// </summary>
+        public IncompleteMigrationExpressionException(string message)
+            : base(message)
+        { }
+    }
+}
diff --git a/src/Umbraco.Core/Migrations/MigrationBase.cs b/src/Umbraco.Core/Migrations/MigrationBase.cs
index 9fbee0ed92..58edcae80a 100644
--- a/src/Umbraco.Core/Migrations/MigrationBase.cs
+++ b/src/Umbraco.Core/Migrations/MigrationBase.cs
@@ -62,42 +62,65 @@ namespace Umbraco.Core.Migrations
         /// </summary>
         protected Sql<ISqlContext> Sql(string sql, params object[] args) => Context.SqlContext.Sql(sql, args);
 
-        /// <inheritdoc />
+        /// <summary>
+        /// Executes the migration.
+        /// </summary>
         public abstract void Migrate();
 
+        /// <inheritdoc />
+        void IMigration.Migrate()
+        {
+            Migrate();
+
+            // ensure there is no building expression
+            // ie we did not forget to .Do() an expression
+            if (Context.BuildingExpression)
+                throw new IncompleteMigrationExpressionException("The migration has run, but leaves an expression that has not run.");
+        }
+
+        // ensures we are not already building,
+        // ie we did not forget to .Do() an expression
+        private T BeginBuild<T>(T builder)
+        {
+            if (Context.BuildingExpression)
+                throw new IncompleteMigrationExpressionException("Cannot create a new expression: the previous expression has not run.");
+            Context.BuildingExpression = true;
+            return builder;
+        }
+
         /// <summary>
         /// Builds an Alter expression.
         /// </summary>
-        public IAlterBuilder Alter => new AlterBuilder(Context);
+        public IAlterBuilder Alter => BeginBuild(new AlterBuilder(Context));
 
         /// <summary>
         /// Builds a Create expression.
         /// </summary>
-        public ICreateBuilder Create => new CreateBuilder(Context);
+        public ICreateBuilder Create => BeginBuild(new CreateBuilder(Context));
 
         /// <summary>
         /// Builds a Delete expression.
         /// </summary>
-        public IDeleteBuilder Delete => new DeleteBuilder(Context);
+        public IDeleteBuilder Delete => BeginBuild(new DeleteBuilder(Context));
 
         /// <summary>
         /// Builds an Execute expression.
         /// </summary>
-        public IExecuteBuilder Execute => new ExecuteBuilder(Context);
+        public IExecuteBuilder Execute => BeginBuild(new ExecuteBuilder(Context));
 
         /// <summary>
         /// Builds an Insert expression.
         /// </summary>
-        public IInsertBuilder Insert => new InsertBuilder(Context);
+        public IInsertBuilder Insert => BeginBuild(new InsertBuilder(Context));
 
         /// <summary>
         /// Builds a Rename expression.
         /// </summary>
-        public IRenameBuilder Rename => new RenameBuilder(Context);
+        public IRenameBuilder Rename => BeginBuild(new RenameBuilder(Context));
 
         /// <summary>
         /// Builds an Update expression.
         /// </summary>
-        public IUpdateBuilder Update => new UpdateBuilder(Context);
+        public IUpdateBuilder Update => BeginBuild(new UpdateBuilder(Context));
     }
 }
diff --git a/src/Umbraco.Core/Migrations/MigrationContext.cs b/src/Umbraco.Core/Migrations/MigrationContext.cs
index d0802c813d..da454fab03 100644
--- a/src/Umbraco.Core/Migrations/MigrationContext.cs
+++ b/src/Umbraco.Core/Migrations/MigrationContext.cs
@@ -4,20 +4,33 @@ using Umbraco.Core.Persistence;
 
 namespace Umbraco.Core.Migrations
 {
+    /// <summary>
+    /// Represents a migration context.
+    /// </summary>
     internal class MigrationContext : IMigrationContext
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MigrationContext"/> class.
+        /// </summary>
         public MigrationContext(IUmbracoDatabase database, ILogger logger)
         {
             Database = database ?? throw new ArgumentNullException(nameof(database));
             Logger = logger ?? throw new ArgumentNullException(nameof(logger));
         }
 
+        /// <inheritdoc />
         public ILogger Logger { get; }
 
+        /// <inheritdoc />
         public IUmbracoDatabase Database { get; }
 
+        /// <inheritdoc />
         public ISqlContext SqlContext => Database.SqlContext;
 
+        /// <inheritdoc />
         public int Index { get; set; }
+
+        /// <inheritdoc />
+        public bool BuildingExpression { get; set; }
     }
 }
diff --git a/src/Umbraco.Core/Migrations/MigrationExpressionBase.cs b/src/Umbraco.Core/Migrations/MigrationExpressionBase.cs
index 4c5a1f1aa7..6ac92a07aa 100644
--- a/src/Umbraco.Core/Migrations/MigrationExpressionBase.cs
+++ b/src/Umbraco.Core/Migrations/MigrationExpressionBase.cs
@@ -50,6 +50,7 @@ namespace Umbraco.Core.Migrations
             if (_executed)
                 throw new InvalidOperationException("This expression has already been executed.");
             _executed = true;
+            Context.BuildingExpression = false;
 
             var sql = GetSql();
 
diff --git a/src/Umbraco.Core/Migrations/Upgrade/UmbracoPlan.cs b/src/Umbraco.Core/Migrations/Upgrade/UmbracoPlan.cs
index fb0f18700c..1edd54a510 100644
--- a/src/Umbraco.Core/Migrations/Upgrade/UmbracoPlan.cs
+++ b/src/Umbraco.Core/Migrations/Upgrade/UmbracoPlan.cs
@@ -119,7 +119,7 @@ namespace Umbraco.Core.Migrations.Upgrade
             Chain<RenameTrueFalseField>("{517CE9EA-36D7-472A-BF4B-A0D6FB1B8F89}"); // from 7.12.0
             Chain<SetDefaultTagsStorageType>("{BBD99901-1545-40E4-8A5A-D7A675C7D2F2}"); // from 7.12.0
             //Chain<UpdateDefaultMandatoryLanguage>("{2C87AA47-D1BC-4ECB-8A73-2D8D1046C27F}"); // stephan added that one = merge conflict, remove
-            
+
             Chain<FallbackLanguage>("{8B14CEBD-EE47-4AAD-A841-93551D917F11}"); // add andy's after others, with a new target state
             From("{CF51B39B-9B9A-4740-BB7C-EAF606A7BFBF}") // and provide a path out of andy's
                 .CopyChain("{39E5B1F7-A50B-437E-B768-1723AEC45B65}", "{BBD99901-1545-40E4-8A5A-D7A675C7D2F2}", "{8B14CEBD-EE47-4AAD-A841-93551D917F11}"); // to next
@@ -141,6 +141,7 @@ namespace Umbraco.Core.Migrations.Upgrade
             Chain<AddLogTableColumns>("{8804D8E8-FE62-4E3A-B8A2-C047C2118C38}");
             Chain<DropPreValueTable>("{23275462-446E-44C7-8C2C-3B8C1127B07D}");
             Chain<DropDownPropertyEditorsMigration>("{6B251841-3069-4AD5-8AE9-861F9523E8DA}");
+            Chain<TagsMigrationFix>("{EE429F1B-9B26-43CA-89F8-A86017C809A3}");
 
             //FINAL
 
diff --git a/src/Umbraco.Core/Migrations/Upgrade/V_8_0_0/TagsMigration.cs b/src/Umbraco.Core/Migrations/Upgrade/V_8_0_0/TagsMigration.cs
index 89cb7e74ac..5dc5e0b6fe 100644
--- a/src/Umbraco.Core/Migrations/Upgrade/V_8_0_0/TagsMigration.cs
+++ b/src/Umbraco.Core/Migrations/Upgrade/V_8_0_0/TagsMigration.cs
@@ -18,7 +18,9 @@ namespace Umbraco.Core.Migrations.Upgrade.V_8_0_0
 
             // kill unused parentId column
             Delete.ForeignKey("FK_cmsTags_cmsTags").OnTable(Constants.DatabaseSchema.Tables.Tag).Do();
-            Delete.Column("ParentId").FromTable(Constants.DatabaseSchema.Tables.Tag);
+            Delete.Column("ParentId").FromTable(Constants.DatabaseSchema.Tables.Tag).Do();
         }
     }
+
+    // fixes TagsMigration that... originally failed to properly drop the ParentId column
 }
diff --git a/src/Umbraco.Core/Migrations/Upgrade/V_8_0_0/TagsMigrationFix.cs b/src/Umbraco.Core/Migrations/Upgrade/V_8_0_0/TagsMigrationFix.cs
new file mode 100644
index 0000000000..4ee95c9f58
--- /dev/null
+++ b/src/Umbraco.Core/Migrations/Upgrade/V_8_0_0/TagsMigrationFix.cs
@@ -0,0 +1,16 @@
+namespace Umbraco.Core.Migrations.Upgrade.V_8_0_0
+{
+    public class TagsMigrationFix : MigrationBase
+    {
+        public TagsMigrationFix(IMigrationContext context)
+            : base(context)
+        { }
+
+        public override void Migrate()
+        {
+            // kill unused parentId column, if it still exists
+            if (ColumnExists(Constants.DatabaseSchema.Tables.Tag, "ParentId"))
+                Delete.Column("ParentId").FromTable(Constants.DatabaseSchema.Tables.Tag).Do();
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/Umbraco.Core/Umbraco.Core.csproj b/src/Umbraco.Core/Umbraco.Core.csproj
index 6762dc508c..2bfa52a80a 100644
--- a/src/Umbraco.Core/Umbraco.Core.csproj
+++ b/src/Umbraco.Core/Umbraco.Core.csproj
@@ -335,6 +335,7 @@
     <Compile Include="Logging\Serilog\Enrichers\Log4NetLevelMapperEnricher.cs" />
     <Compile Include="Manifest\ContentAppDefinitionConverter.cs" />
     <Compile Include="Manifest\ManifestContentAppDefinition.cs" />
+    <Compile Include="Migrations\IncompleteMigrationExpressionException.cs" />
     <Compile Include="Migrations\MigrationBase_Extra.cs" />
     <Compile Include="Migrations\Upgrade\V_7_10_0\RenamePreviewFolder.cs" />
     <Compile Include="Migrations\Upgrade\V_7_12_0\AddRelationTypeForMediaFolderOnDelete.cs" />
@@ -367,6 +368,7 @@
     <Compile Include="Migrations\Upgrade\V_8_0_0\SuperZero.cs" />
     <Compile Include="Migrations\Upgrade\V_8_0_0\TagsMigration.cs" />
     <Compile Include="Migrations\Upgrade\V_8_0_0\FallbackLanguage.cs" />
+    <Compile Include="Migrations\Upgrade\V_8_0_0\TagsMigrationFix.cs" />
     <Compile Include="Migrations\Upgrade\V_8_0_0\UpdateDefaultMandatoryLanguage.cs" />
     <Compile Include="Migrations\Upgrade\V_8_0_0\UserForeignKeys.cs" />
     <Compile Include="Models\AuditEntry.cs" />
diff --git a/src/Umbraco.Tests/Migrations/MigrationTests.cs b/src/Umbraco.Tests/Migrations/MigrationTests.cs
index b408d351d7..d06cf2244e 100644
--- a/src/Umbraco.Tests/Migrations/MigrationTests.cs
+++ b/src/Umbraco.Tests/Migrations/MigrationTests.cs
@@ -1,16 +1,19 @@
 using System;
 using System.Data;
+using Moq;
+using NUnit.Framework;
 using Semver;
 using Umbraco.Core.Events;
-using Umbraco.Core.Logging;
 using Umbraco.Core.Migrations;
 using Umbraco.Core.Migrations.Upgrade;
 using Umbraco.Core.Persistence;
 using Umbraco.Core.Scoping;
 using Umbraco.Core.Services;
+using ILogger = Umbraco.Core.Logging.ILogger;
 
 namespace Umbraco.Tests.Migrations
 {
+    [TestFixture]
     public class MigrationTests
     {
         public class TestUpgrader : Upgrader
@@ -67,5 +70,68 @@ namespace Umbraco.Tests.Migrations
             public IScopeContext Context { get; set; }
             public ISqlContext SqlContext { get; set;  }
         }
+
+        [Test]
+        public void RunGoodMigration()
+        {
+            var migrationContext = new MigrationContext(Mock.Of<IUmbracoDatabase>(), Mock.Of<ILogger>());
+            IMigration migration = new GoodMigration(migrationContext);
+            migration.Migrate();
+        }
+
+        [Test]
+        public void DetectBadMigration1()
+        {
+            var migrationContext = new MigrationContext(Mock.Of<IUmbracoDatabase>(), Mock.Of<ILogger>());
+            IMigration migration = new BadMigration1(migrationContext);
+            Assert.Throws<IncompleteMigrationExpressionException>(() => migration.Migrate());
+        }
+
+        [Test]
+        public void DetectBadMigration2()
+        {
+            var migrationContext = new MigrationContext(Mock.Of<IUmbracoDatabase>(), Mock.Of<ILogger>());
+            IMigration migration = new BadMigration2(migrationContext);
+            Assert.Throws<IncompleteMigrationExpressionException>(() => migration.Migrate());
+        }
+
+        public class GoodMigration : MigrationBase
+        {
+            public GoodMigration(IMigrationContext context)
+                : base(context)
+            { }
+
+            public override void Migrate()
+            {
+                Execute.Sql("").Do();
+            }
+        }
+
+        public class BadMigration1 : MigrationBase
+        {
+            public BadMigration1(IMigrationContext context)
+                : base(context)
+            { }
+
+            public override void Migrate()
+            {
+                Alter.Table("foo"); // stop here, don't Do it
+            }
+        }
+
+        public class BadMigration2 : MigrationBase
+        {
+            public BadMigration2(IMigrationContext context)
+                : base(context)
+            { }
+
+            public override void Migrate()
+            {
+                Alter.Table("foo"); // stop here, don't Do it
+
+                // and try to start another one
+                Alter.Table("bar");
+            }
+        }
     }
 }