Files updated during first migration so migration will not complete

I have a migration that gets stuck (see attached image). I'm using the standard declaration in hook_register_migrations():

// File Migration for Image Attach Images
  $file_arguments = $common_arguments + array(
    'machine_name' => 'ImageAttachFileMigration',
    'description' => t('Import Drupal 6 File references to be used by content types that used image attach in D6.'),
    'user_migration' => 'RanchoUserMigration', // User migration must be run first so the ownership can be set.
    'default_uid' => 1,  // Seems appropriate
    'source_dir' => 'http://example.com/',  // See https://www.drupal.org/node/1540106
  );
  Migration::registerMigration('DrupalFile6Migration', $file_arguments['machine_name'], $file_arguments);  

This seems like a bug to me.

As a side note:
For the past couple years, something has confounded me about Drupal Migrate: there is a --update option and a --needs-update option when using Drush, but Migrate seems to update on it's own without using either of these options.

Seems like we need a --no-update option IMO.

Update: I stopped the migration in the UI and moved to Drush. I am seeing an interesting pattern emerge: seems like if you use the --limit option, the migration will update however many nodes have been imported to date, then continue importing new nodes, which is a slight improvement; see below.

Update 2: seems if you set the exact limit, everything imports in one shot. If you just run the migrate without setting the limit, starting after you hit whatever limit (one minute?), the updates start, and after awhile, you only get updates.

Update 3: Edit removed this update as it was not relevant

Also noted that, say you only have one new item to be imported, but several updates, if you run "drush mi", the updates happen first. I'm sure I'm not the only person that finds this behavior totally undesirable, especially without specifying the --update option.