I don't have much experience with spitting on the crowd but I can comment on mistakes with two words: Poker face. If you play every single note wrong but you look like you are playing every note perfectly, you'll have a slightly confused audience with more supporters than haters. If you play 95% of the notes perfectly and look like you are playing every note perfectly then as far as anybody is concerned, you are playing every note perfectly. To make this work you have to have a "poker face" on all the time. Body language, spoken language, restarting the music are all examples of "tells"; the audience now knows you have a terrible "hand of cards." If you perform horribly, but with confidence, you may have a "Royal Flush" and the audience will likely assume you do. PS - If the majority of people know the song, such as Sweet Home Alabama, no poker face in the world will work.