Slightly different (perhaps?) from being tone deaf in the sense that you can't sing at all, an interesting phenomenon I've encountered is where you have someone who is actually a very good singer, but they seem to have trouble finding the right key, but once they find it they're OK. I've known a few of these over the years. I used to accompany one guy who would often start in the wrong key unless I played the actual tune in the intro, instead of just the chords. I guess this was a kind of "chord deafness" - has anyone else encountered this?