I like the ones which back fire on the perpetrator.

Some years ago I was with a group of teaching colleagues and forty or so youngsters on an outward bound week. We were playing silly games with water filled balloons and were being plagued by midges. Our young male teacher had some Tiger Balm with him and he tried to convince me and a female colleague that if he dabbed it on our pulse points it would act as a repellant. We told him where to get off but he already had it on his fingers. He rubbed his hands on his shorts and continued to play with the kids. About 20 minutes later he disappeared to answer the call of nature. His face when he returned was a picture - he was squirming for the rest of the evening.
He really should have washed his hands before touching any delicate skin!