Setting this to major because it is a common situation that totally aborts the migration process.
To recreate the problem, create several view modes in the source database. Set some fields to be hidden in some of the view modes. Then attempt a migration. When you get to the migration process for d6_field_formatter_settings, the migration will stop with a message like:
Failed to lookup array (
0 => 'text',
1 => 'hidden',
) in the static map.
I think hidden fields should be treated like excluded fields and just skipped when processing view modes. I'm attaching a patch that would do that. It allowed me to get through the migration without errors when I had hidden fields. And in the resulting D8 site all the fields looked right, hidden in the view modes they should be hidden on.
This was one cause of problems noted in https://www.drupal.org/node/2361401.