As Nigel says, if you make a little mistake just ignore it. They aren't listening that closely.
If you make a BIG mistake, I make an announcement at the end of the song:
"Sometime during the evening you may THINK you hear me make a mistake. It's true. I like to please everyone and some people are always looking for something wrong, so I will throw in an INTENTIONAL mistake so that I'm sure I please everyone."
DonM