What happened was (I'm pretty sure):

Mississippitom never actually registered. He likely thought he was registering by changing all of Boberts information to his because Bobert was still logged in.

Bobert then couldn't find his name because Tom had changed it.
Bobert started a new membership, but

All of Bobert's previous postings are showing up as mississippitom's, all of the PMs he sent and received that he didn't delete are now Tom's.

The best solution (I believe) is to

1. Get Tom to log out
2. Tom or Joe can send Bobert the current password. He undoubtedly used a real e-mail in the SpongeBob SidewalkPants account.
3. Explain to Bobert how to change the info in the accidentally hi-jacked membership back to his.
4. Explain to Tom how to register. He likey can figure this out since he did such a great job altering Bobert. OR you can have him stroll on over to the Sidewalk account.
5. Perhaps an Uber-Mudgeek could sort out the messages to the proper accounts.