Not a blooper, but apparently in every Beatles album cover, there is something "wrong" with the picture. For example in "Abbey Road" Paul is not wearing shoes or a tie and in "Sgt. Peppers'" cover, Paul has (I beleive) a black rose, which is a sign of death.
O ya, in the Nirvana song "Dumb" on the unplugged cd, Kurt Cobain goes back to puberty for a second. This happens at 38 seconds into the song.