I fiddle with an unused channel - 9 times out of 10... drummers wife thinks I fixed or worked on her complaint. Dunno. Works for her though.
I also buy myself time with "hey give me 10 mins, I am working on a problem that doesn't exist..." they usually forget or environment changes, blah blah, works well.
Yup, used both of these a time or two (although 90% of complaints I hear come in during the first 4 bars of the first song and I really am working on an issue).
My favorite complaint is when the GF/mom/pita comes up before they even start playing to be 'sure I can get the sound they want'. This is (of course) right after I've gone from a 3pc rock band to a 5pc funk-rock band and just getting back to the board and just after starting to do sound check. I typically check new mic's first (which of course includes the guy who 'everyone' wants to hear), and listen to the compaint while un-muting all the channels and give em a go signal. Of course I forgot the new channel and that fancy sax solo is heard by no-one... including myself who is now working on getting the vocal up over their crappy amps since they didnt check at level in the first place. ..... FF to the end of the night (be sure the GF/mom/pita is nearby).... I apologize to the individual who I forgot to get in the mix, but make sure I let them know that I was fielding an issue that their (GF/mom/pita) was bringing up, and I missed their channel because of it........ makes for an embarassing moment for the PITA, and a happy comical ending for me. Have fun with it........