The worst, musically, include some Europeans singing American folk music with what they think is an American accent. Which makes me worry about my own singing of Scottish music. I'm not trying to pretend I have a Scottish accent (I lived there for only a short time), but with much Scottish folk music you have to use the old Scottish pronunciation or the lines just won't scan. And then too, I usually sing songs the way I heard them originally, and I usually learned them from Scottish singers, which means... well, I'm sure no one's going to think I'm actually native Scottish and I worry it may sound as though I'm trying to pretend I am.

And again then there's just plain bad pronunciation, like Paul McCartney in the early Beatles recording of Till There Was You...

There were birds in the air,
But I never SORE them winging
No, I never SORE them at all,
Till there was you.