My favourite was when I was playing using Moogs (yes this was a few years back) suddenly a little head appeared above my rig and a small voice asked "What happens if you twiddle these knobs, mister?", whilst busily twiddling the knobs to see for himself. Luckily our singer was a teacher who quickly removed the offender from the stage by his ear (I guess you wouldn't be able to do that these days). She didn't break stride, still singing her heart out at the top of her voice!