I've noticed it before, particularly the Debbie Reynolds bit you mentioned--though, for some reason, it never occurred to me to keep track(I guess I always thought I'd remember)--

You're right, it seems like everyone would know about it-- movies are notorious for inside jokes--though it is completely possible that on the set, the music was in a different key than in the finished movie--

At any rate, I have also noticed other related things, such as guitars being played in a key different than the one you hear, and say, a trumpet solo coming out of a saxophone, along with the standard routine where someone sings while playing the piano, then gets up to dance while the piano still plays--