Where to begin...
I won't name my absolute worst, since I'm still coming to terms with it, so I'll explain something that was bad enough...
This was much earlier on in 2005. I was teaching the beginner class, which was almost all children of varying ages (5-12). Three boys (triplets, actually) were constantly running off the floor to go to the bathroom, and when one would leave the floor, the other two would take off running after him.
After I warned them several times to stay on the floor, and they still wouldn't listen, I finally put my foot down, and told the class in a stern tone, that someone may only leave the floor for a bathroom break if it is an absolute emergency.
Well, that stopped the chaos for a bit. When the class was about halfway over, I was having the class do some kata as a whole group. I told them to freeze halfway through, so I could point out some stance corrections, and even told them to use their mirror image as a guide. While my back was turned to them as I was pointing at the mirror, there was suddenly this unified cry of "EEEEEEEWWWWWWWWW!!!!" by many of the children there.
I turned around, and saw that everyone had scattered to the edges of the dojo floor, except for one boy who was rather ashamed of himself at having just puked out his sausage biscuit, hash browns, and orange juice from his breakfast that he had eaten just before class. At that point, I simply told the whole class to sit down at the edges, and be silent, while I grabbed the cleaning supplies, and eventually getting the place clean. Unfortunately, the big pile of puke that he left in the middle of the dojo floor wasn't nearly as large as the puke he spewed all over the floor leading to the bathroom, as he had a second eruption.
After I had scrubbed down the floors, and even disinfected it, many of the children who stayed for the next class absolutely refused to step anywhere near those areas. I told them "Look! It's perfectly clean now. I'll even show you!" as I stepped in that spot, as this generated and even louder "EEEEEEWWWWWW!"
I just wanted that day to be over before I lost my sanity.
A few days later, when I was teaching another class, the puking boy came up to me and started profusely apologizing, and he was nearly in tears. I told him that it was OK, and that accidents happen. I then found out that my lecture on only going to the bathroom during class for emergencies had scared the poor kid, as he didn't think that having to puke was an emergency.
The next time I taught classes, I started out with another lecture, clarifying what constitutes an emergency.
One of the parents who had witnessed the whole thing swore that she saw a few more grey hairs in my head after that incident.