I actually was accused of smoking on a plane (I don't smoke!) by a FA. I was returning from the bathroom and a FA came to my seat and loudly accused me infront of the whole plane of smoking. I was more than a little put off (and embarassed) and denied it. She then came back with a half smoked cigarette from the trash and waved it all over the cabin (and made the whole cabin smell) accusing me of smoking.

I would up offering to let me smell my fingers at which point she realized she went too far. I got her information and reported her to AA corporate (and got a substantial travel voucher).