diff -u b/core/modules/comment/migration_templates/d7_comment_type.yml b/core/modules/comment/migration_templates/d7_comment_type.yml
--- b/core/modules/comment/migration_templates/d7_comment_type.yml
+++ b/core/modules/comment/migration_templates/d7_comment_type.yml
@@ -12,6 +12,15 @@
     plugin: static_map
     source: bundle
     bypass: true
+    # If the destination site installed the Standard profile, there will
+    # already be a 'comment' comment type, and the Article node type will be
+    # using it. To avoid data integrity issues, we want to reuse that comment
+    # type and the associated field. If the destination site did NOT install
+    # Standard, this will create the 'comment' comment type. The Forum module,
+    # meanwhile, also provides its own comment type (comment_forum), which we
+    # want to reuse if it exists, for the same reason that we want to reuse
+    # the 'comment' comment type if it exists.
+    # See https://www.drupal.org/node/2853872 for more information.
     map:
       comment_node_article: comment
       comment_node_forum: comment_forum
diff -u b/core/modules/field/migration_templates/d7_field_formatter_settings.yml b/core/modules/field/migration_templates/d7_field_formatter_settings.yml
--- b/core/modules/field/migration_templates/d7_field_formatter_settings.yml
+++ b/core/modules/field/migration_templates/d7_field_formatter_settings.yml
@@ -26,6 +26,10 @@
       plugin: skip_on_empty
       method: row
   entity_type: entity_type
+  # The bundle needs to be statically mapped in order to support comment types
+  # that might already exist before this migration is run. See
+  # core/modules/comment/migration_templates/d7_comment_type.yml and
+  # https://www.drupal.org/node/2853872 for more information.
   bundle:
     plugin: static_map
     source: bundle
diff -u b/core/modules/field/migration_templates/d7_field_instance.yml b/core/modules/field/migration_templates/d7_field_instance.yml
--- b/core/modules/field/migration_templates/d7_field_instance.yml
+++ b/core/modules/field/migration_templates/d7_field_instance.yml
@@ -11,6 +11,10 @@
 process:
   entity_type: entity_type
   field_name: field_name
+  # The bundle needs to be statically mapped in order to support comment types
+  # that might already exist before this migration is run. See
+  # core/modules/comment/migration_templates/d7_comment_type.yml and
+  # https://www.drupal.org/node/2853872 for more information.
   bundle:
     plugin: static_map
     source: bundle
diff -u b/core/modules/field/migration_templates/d7_field_instance_widget_settings.yml b/core/modules/field/migration_templates/d7_field_instance_widget_settings.yml
--- b/core/modules/field/migration_templates/d7_field_instance_widget_settings.yml
+++ b/core/modules/field/migration_templates/d7_field_instance_widget_settings.yml
@@ -26,6 +26,10 @@
     -
       plugin: skip_on_empty
       method: row
+  # The bundle needs to be statically mapped in order to support comment types
+  # that might already exist before this migration is run. See
+  # core/modules/comment/migration_templates/d7_comment_type.yml and
+  # https://www.drupal.org/node/2853872 for more information.
   bundle:
     plugin: static_map
     source: bundle