Worst time was at a nursing home with the Growing / Shrinking head illusion spiral. I was in front of about 50 people who were 80 or so. Apparently...I did not know this, but older people's eyes cannot "focus" on the spinning of the spiral for the required amount of 30 seconds to make the illusion happen. I was waiting for the laughter...as it normally comes with this effect...and....nothin'....silence...crickets... So...I try it again....nothing. OH well...and that was my opener. Boy could I feel the sweat running down my forehead. "And for my next effect...." Other embarrassing moment was at a different gig and when fanning the entire invisible deck at chest height and the audience was seated and saw how it worked....man did I feel dumb. Point taken and it has never happened again.