Is it OK for audience members to start joining in uninvited during a song?

It's annoying enough when someone nearby starts doing it in an amplified concert setting but I've also seen it happen to session singers as to well known performers in folk clubs.

I recently saw a someone singing a reasonably familiar song but with adventurous phrasing who was completely flummoxed when someone insisted on accompanying her word for word but totally out of sync.

I suppose some people get carried away and do it without thinking but I suspect it's mostly show-offs who want everyone to notice that they know the words.