So far I've noticed it often happens when a sim is watching a skill related television channel (ie. cooking) and then another sim hijacks the tv to play video games. The sim fails to realize that it's no longer watching tv and continues to skill that specific ability forever. Even when the blue bar doesn't show up, that one skill is still increasing when you look in the skill tab.

I got cooking, fishing, and gardening maxed that way  

After lurking a while, I've finally found something amusing enough to be worth posting.

Granny gaining fishing skill while asleep. Talk about multi-tasking.

This is the first Sims 3 bug I've found that actually does something useful