The worst is when you're playing a PSR740 or a PSR2000 and you think you have the freeze button on so you can press a registration button and only the voice changes. Whoops, your theme song from Dr. Zivago suddenly turns into a 212 bpm Dixieland!
On the PSR740 there is a horrible feature that the freeze button does not freeze the transpose. So if you have a song transposed and you press a registration button, the song gets transposed to 0. Oops!

Larry