I altered the Drupal 7 user registration form to be multi-step. On the first step is the username and email textfields. On the second step is all the extra user fields by setting their #access property to false on the first step and true on the second step. That works fine. If I move one of the fields to the first step, hit next, then submit the form or hit previous, this error appears. I'm having a real difficult time trying to understand why this is occurring because I'm not very familiar with working with fields in forms.
Line 41 of field.default.inc appears in field_default_extract_form_values().
Edit: I narrowed the issue down to something with checkbox fields in the first step. I removed the checkbox field from the first step and replaced it with a textfield field. No error appears. So there appears to be a bug with Drupal core handling of checkbox fields on a multi-step form. I will update as I progress.
Update: See http://drupal.org/node/1671422
