I’ve been writing migration code for my wife’s WordPress blog to Drupal 8, using the migrate framework. This framework attempts to track if a particular migration is currently running, but does not always unset that flag if it crashed with a PHP fatal error (which will happen when you are writing new code.) Subsequent attempts to run the migration will produce “Migration your_migration_name is busy with another operation: Importing”.
To unstick it in this case, you can run the following in drush:
drush php-eval 'var_dump(Drupal::keyValue("migrate_status")->set('your_migration_name', 0))'