My Divemastering (is that a word?) experience is pretty small right now, and will probably stay that way, but the worst I ever had to deal with (and this is nothing compared to some of the stories I've read in here) was actually a couple of different people in an AOW class.
One was a woman, who was just a total *****. On the day of the navigation\s&r\night dives, I saw her struggling to get into her BC. She was partnered up with two other women, both of whom were busy helping each other, so I asked her if she needed a hand. She tore right into me, exclaiming that she could do it herself and needed no help from me or from anybody. To her credit, she did get all suited up by herself, but it took her quite a long time. The instructor, in an effort to improve her trim a bit (she had the tendancy to actually walk across the bottom - and our local lakes are VERY silty) had asked her to not use her ankle weights for this dive. Rather than throw a tantrum this time, she decided to go the passive-aggressive route and remove the ankle weights. Of course everytime she tried to descend she would just roll over onto her side and complain really loudly that she needed her ankle weights. Well, we finally gave in and went to get her ankle weights for her. Funny thing... next to her ankle weights was the integrated weights from her BC that she had forgotten to insert. Probably wouldn't have happened if she had accepted some help in gearing up.
The other oh-so-fun student was a guy who "had been diving for 10 years". I've come to realize that anytime a student says they've been diving for 10 years, it means that they are a horrible diver. I think the amount of diving in these claims is directly proportional to the general suckiness of the divers skill. And this guy was no exception. He was paired up with the only other guy in the class, who was actually a really good diver. Anyway, it was time for the night dive, and the instructor took the class (3 women as a buddy team, and the two guys as the other buddy team) on a night tour with me trailing behind keeping an eye on things. They got to the turnaround point and the instructor told them all to ascend. At the surface he directed them to take a compass bearing back to our start point and to lead themselves on. This is where the fun begins.
The 5 students descend, only one of the girls is having some trouble equalizing. Her buddies (one of which is the woman I talked about before) are oblivious to her plight and just head on out, never once realizing that she wasn't behind them. Well, the instructor chased after them to ask about where their buddy was, but I never saw them again on that dive, so told the girl to just buddy up with the two guys for the rest of the dive or until her two buddies returned.
Ok, I'm rambling... sorry. Anyway, the two guys and the girl head off, and I'm following. Suddenly the 10-year veteran just stops and turns to me and asks me where his buddy (the other guy) is. Well, his buddy was directly above him, so I pointed up. What's he do? He signales OK, and starts to ascend. I stop him and point up at his buddy. So again, he starts to ascend. I stop him again, and this time signal LOOK. UP. He starts to ascend. I stop him, grab his head and tilt it up. Light dawns and he suddenly is happy to have finally found his buddy. So they start heading to shore again.
Inexplicably, this guy's compass slips into some alternate universe where the magnetic poles are different from good ol' Earth's, and he just turns 90 degrees to the left and just tears off as fast as he can. Now I'm stuck with a descision. Do I stay with the two other divers, or do I try and save this guy from himself? Well, the other two were decent divers, so I decided to leave them and chase this guy. He's going as fast as he can, and I was just able to catch him. My hand was just about to grab his fin, when his compass miraculously returned to our reality and started pointing in the right direction again. I seriously wonder how this guy hasn't gotten himself killed in his glorious 10 year career.