Gameplayer.au has a (spoiler-free) warning for Fallout 3 players, as they've identified a point that will lock players into the end-game, after which it will be game over man, as it eliminates the possibility of experiencing unexplored areas of the post-apocalyptic Washington D.C. They provide advice on managing your save games to ensure you do not get caught in this inadvertent "trap" before getting the full game experience, which is probably sage advice to follow for handling save games in general.