1) log in as an admin with "administer groups" permission
2) in a group where the logged in user is not a member, create new content using a required group audience field with entity prepopulate via the URL.
3) Despite not being a member, the group appears in the 'default' field as part of the combo audience field for group administrators.
5) Click 'upload' on a file/image field within the group content node form. The default field will still show the prepopulated group in 'your groups'. (If no ajax submit buttons are clicked, saving works.)
6) Attempt to save.
7) Your groups is cleared, and the node form throws the following error: "An illegal choice has been detected. Please contact the site administrator." - from line 1388 in includes/form.inc - because the value of the prepopulated group is no long in the options for 'your groups'.
8) This error is compounded because after the error, it's no longer possible to select the prepopulated group in EITHER 'your groups' or 'other groups'. If selected in 'other groups', the referenced group throws "The referenced group (@type: @id) is invalid." from OgBehaviorHandler.class.php because the prepopulated value is not listed in the results from entityreference_get_selection_handler() any longer.
This issue is specific to fields with 'skip_perm' configured and users with the matching permission. It occurs because entityreference_prepopulate_field_attach_form() has a continue; statement for users with 'skip permissions' BEFORE executing the code that would store the value in the $form_state.