In field_features_rebuild() the function field_create_instance() is called which for the OG module can cause it to create a new field (Group membership on the user entity).
In this case the $existing_fields variable which Features use to choose whether to create or to update a field is no longer accurate.
This causes it to attempt to create the field that OG just created if it is exported, rather than update it.

I'm on the fence about this one.
Perhaps modules should not be creating fields and instances in hook_field_create_field() and hook_field_create_instance() but this seems like a rather arbitrary restriction just to make sure Features works properly.
Perhaps Features should use field_read_field() inside the loop. It does add an extra query per field but does not have the overhead of field_info_field() or resetting caches (both of which are not needed when simply checking for existence of a field).

Any other ideas?