It occurs when the game tries to autosave or the user manually saves, allegedly. EA can provide no reason for the bug but continue "to look into it."
The Sims 3 launched for Xbox 360 and PS3 last October and only recently the bug has cropped up it would seem. Not all gamers are affected but EA and the community are obviously worried it went unnoticed for so long.
Currently there are no words of caution to follow as what triggers the crippling defect remains a mystery. The best thing right now is to inform EA of your experience should it happen. Check out thisfor more information.