"One of the worst examples of bad manners I have witnessed is when a song is sung in a music session and a musician (who may be listening to a song for the first time), insists on joining in." Jim Carroll.

I'm a contemporary singer-guitarist who frequents folk singarounds and sessions, and this sometimes happens to me, even though most of my material is not widely known. I'm quite happy when people do know the song, indeed I sometimes invite them, but in many cases, although they have no chance of knowing or even busking along with it, some will persist anyway. It puts me off my stride and I have on more than one occasion finished a song early as a result, and once ground to a halt - irritating for me and those who were listening.