The beep that happens right before the Entr'acte on the awfully mastered 'Mamma Mia!' CD. (Gotta be the cue, or something right?)
There's a few on the International recording of 'Les Mis' the little girl sings her "Castle on a Cloud" line in reverse.
"Lots of children to see" instead of "castles to see".
Hamlet's father.