I saw Judy Collins in the late 1960's at the Albert Hall and she completely lost the words and had to get the drummer to remind her before going on.

Not one of his own songs but Dick Gaughan responded to a request when I last saw him to sing Ewan MacColl's 'Fathers Song' and left out a verse - though he did say when I asked him that he hadn't sang it for a long time. Being as I thought it kind of him to bother to do it I didn't point it out to him :)