Another situation is the user's avatar. I have that set for "show" in the registration settings but in the actual registration it shows avatar but does not let them add one during the time of registration. They have to go back to "my stuff" then "profile settings" to go back and add.
Ahaaa, I can help for a change! I had the same problem, but after puzzling over it for some time finally twigged what it was. I had 'upload avatar' turned on for everyone EXCEPT Guests. And what are people who haven't yet completed registration? You've got it - Guests.
Turn on 'Guests can upload avatars' and you'll be sorted.