I'm not sure if this has already been posted or not (and I'm too lazy to look), but I've been having an issue with traits.  When the sim doesn't "grow up well", and the game auto-assigns them, about 75% of the time they don't show up.  It will give the whole dialog like "Little Tommy X didn't grow up so well, so he now has the Insane trait".  But the trait will never show up in their list.