First time I played the game, I accidentally bugged the ending by something like this. I entered the Jefferson Rotunda *without* the Brotherhood girl who was unconscious. But once inside, the doors get locked and you're completely stuck.
What do you mean with 'bug abused'? It's been a while since I played Fallout 3, but I simply ran past everything the first time I played it.
I'm new here, BTW, so hello.