One of the problems I've seen is divers overestimating their skill level. Recently I was on a LDS organized trip to Cozumel. One of the divers blew his safety stop on every dive, whacked the coral on each dive, kept bumping into divers as he tried to wedge in to see things and was either over or under weighted on each dive. When anyone complained to him about his conduct, it was always someone else's fault. The dive master gave him too much/too little weight, he wanted to see things so he had to bump into people, weighting problems caused his blown safety stops, etc. And all the while, after diving, he went on and on about his experiences in his long diving career.
I tend to underestimate my capabilities since I don't get to dive as often as I like. I prefer to hang out with the dive master for at least the first day of diving to make sure my weight is correct, I have on the right amount of exposure suit and basically have my head on straight. Since I had not dived in tropical water for some time before the CZM trip, I left my camera in the room the first day of diving just so I wouldn't have too many things to think about. I felt pretty good at the end of our trip when the dive master came up to me and told me that he felt I could do any dive I wanted, and he'd love to dive with me again (and the tips had already been given out anonymously prior to his comments). I suspect that the fact that he and I were pretty much the last two divers out of the water on most of the dives had something to do with his comments.