EDIT: After finally finding a manual on the internet and much trial and error, we managed to get the bolt and trigger assembly back out, then figured out that you need to be pressing in the bolt release when you insert the trigger assembly. Otherwise... it locks up like nobody's business. So problem solved.
My brother just got an M3 Super 90, used off Gunbroker, and with no prior experience with the gun (and no manual, and no manual to be found anywhere on the internet) he disassembled it and cleaned it, mostly pain-free.
While putting it back together, he put the bolt in before the action bar. Now the gun is completely locked up. Nothing we do will get the bolt to come out. We can't get the trigger assembly out (which I guess is what it's getting hung up on) and the bolt catch is stuck in the out position.
I'm about ready to believe that he has created a very unwieldy paperweight and there is no way to get this thing apart short of a saw or a blowtorch.