I've had a few doozies. Luckily never had to clean up the brown stuff, but there have definitely been puddles. Boy, oh boy, have there been puddles. One time it happened during the belt testing. Kudos to the kid becuase he still came back after that mortifying experience.
Most of the adult students are pretty good. Some of the kids though...wow. ADHD to the extreme. The kids white belt class is my own personal hell. Three particular kids, when in the same class, become demons of the tenth order. One day, when there were no assistants to help me, I had a white a belt class of twenty with these three boys in it. I'm still traumatized. One of the boys (who is 4 years old and has no attention span whatsoever - personally I think he is too immature for martial arts) really made my day by calling me a stupid-head in front of the whole class. I think that day ranks as one of my personal worst...ever.
Regarding adults, the worst problem is the chronic gas of a couple of the guys. It's the worst when we do any abdominal conditioning. P-U!