Subject: singers who sing wrong lyrics

I wonder how many professional singers have 'off' days when performing and accidentally sing the wrong lyrics (as opposed to the listener mishearing them)? A relative of mine, while appearing in a long run of Joseph and the Amazing Technicolor Dreamcoat, sang "Egypt with a capital C" (instead of capital E, obviously) and could hear cast members s******ing behind her. In the same show she also sang "No-one had votes or a vote but the king" insread of no-one had rights or a vote..." She also  often had an almost overwhelming urge to sing " there's been a run on margarine" instead of "...run of crazy dreams"