The wonderful Frank Harte was notorious for setting off in the wrong key and having to restart. He was also quite likely to shift key again during the song. Did it affect his ability to engage and entrance an audience? Did it hell!

Even now, three years after Frank's death, if a singer at a session changes key drastically during a song, someone will call out "Good man, Frank!" from the crowd.

It's not a matter of tolerating (or even recommending!) poor musicianship - and the margin for error, so to speak, DOES depend on the circumstances - but it's by no means the most vital part of performing a traditional song.

Regards