I went through all the grueling, grueling trouble of carrying that damn gnome in Half-Life 2 : Episode 2 all the way across the game, and it didn't even give me the achievement for it. Did I put it in the rocket wrong or something?
 
Also, I was 2 antlion grubs away from that 333 kill achievement. 2...
 
Those are now the only achievements I'm missing from that game.