Jamming is about making music with likeminded people. It is about harmony. When someone comes along who is not in time or in tune with the rest of the group then it stops being a jam and starts becoming an irritation for all but the person who has spoilt it for the rest.

I have experienced this at singarounds where a 'musician' has started to accompany me unasked. If they are good, fine with most songs although some are best acapella. When the 'accompaniment' is out of tune or playing twiddly bits to show off I get pissed off and can quite understand how Beer felt on this one. There is one particular mandolin player that I would have loved to tell to shove his instrument right up his arse, but that just isn't done, is it? But why should most people's enjoyment have to be spoilt so that one idiot can feel that they are joining in?