Have you ever discovered any "hidden" sounds in stems, instrumentals or acapellas?

I was listening to Lily Allen's "Air Balloon" and I noticed that, in the chorus, Lily's is in the background singing "air balloon, air balloon" in a high-pitched chipmunk voice.

In the Guitar Hero: Metallica multitracks of "Ace of Spades", Lemmy says an extra lyric: "That's right, babe!" That's because the song was re-recorded for the game, as the original multitracks are lost.

In the instrumental of Crystal Castles' Celestica, a synth is playing the exact same notes Alice Class sings.