The following *spoiler free* public service announcement comes to you from Vault-Tec Industries.
There's radioactive trouble abrewin' in Washington DC fellow gamers, and you all need to be on your guard against a new threat. Now, we know what you're thinking; "Yeah, yeah gameplayer, I've already forty-fived the feral ghouls, beaten the snot out of the Super Mutants, and rail spiked the RadScorpions to the wall. I'm a wasteland owning gunslinger and my 'L33T sk1llz' are enough to make even Mad Max look sad and lax".
Even still, intrepid adventurer, you're still in mortal danger of being thwarted by a foe you'll never see coming, something much, much worse than death: ruining your 20 + game save by completing the final mission in the main quest.
In case you missed the large warning in our Fallout 3 review, if you complete the main quest by walking into the final room of Fallout 3, the game will lock you in there until you do what needs to be done - then it'll boot you back to the title screen. This sucks because there is so much more to explore in the Washington wastes and, unless you've saved beforehand, you'll be forever denied access to the outside world. Unless you start again, but really, who wants to do that crap?
So take the tip, whenever playing Fallout 3 - or any game of epic size and scope for that matter - get in the habit of having three saves; the games Autosave which triggers when you enter a new environment, plus two more that you should save over in sequence. Doing things this way will help ensure that your murderous sprees in Fallout can continue, while preventing you going on an actual murderous spree in the real world when you ruin your save.
Will Bethesda rectify this dreadful oversight? Who knows, but in the meantime, you've all been warned...