Sometimes I wonder if these things happen on purpose... It almost always happens with games that aren't really anticipated. Like, if they don't think a game will have great sales they'll do this, leave it until they notice people are spreading the news, and "fix" it after a little while. Get cheap copies out there, those people who paid next to nothing will want to play it right away because "heh, $3 day one" and they'll spread their opinions of the game. More people talking about it means more exposure. Plus the price error itself was also more exposure.
Or these things just happen.