Right on! I'm lucky that in over 30 years of playing we haven't had much of that. We do get the usual bad titles such as "can you guys play the bullfrog song?" The best way to get rid of a wannabe sit in singer is to tell them you are playing the song in their key but actually play it badly in another key either too high or too low. It helps to add or skip a few beats too although most wannabe singers already do that. Now I would NEVER do these type of things but I've heard rumors that some musicians do.
Tom
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Thanks,
Tom