For some reason when I was playing the first Mass Effect for the first time it stopped giving me achievements after the first one. Now, normally I wouldn't give two shakes of a rat's ass about achievements, but Mass Effect actually gives you some in game bonuses for obtaining them. Despite this, rather than go back and restart after two hours of playing, I decided I didn't want to do that and pressed on and finished the game.
On my 2nd playthrough, it did give me my achievements, and even took into account some of them I didn't get from the first time. Like, it gave me the one for completing the game twice, even though I was getting the one for completing it once at the same exact time.