I have dove with 1 gentleman who, at the time was beginning to show some dangerous tendencies, at least by the accounts of others who had dove with him, or had conversations with him. I should have taken the hint at that time & not gone on that dive, but not having really dove with him before, I decided to give him the benefit of the doubt. We were supposed to, by plan, drop down to the deep part of the quarry. The plan was for us to swim up the slanting road to the shallow end to to look at the attraction, but there were no plans made to penetrate them. As we were making our way along, he suddenly decides to start penetrating the boats, vans & other such attractions down there. I try not get into overhead environments, personally (since I have no training), so I just waited outside. Keep in mind that we are both OW divers of the same level (rescue) After him doing this attraction after attraction, I'm starting to get low on air & pushing close to deco limits. I let him see both my SPG & my computer, so there was no mistaking what I was trying to communicate. I signal to him that it's time to start ending the dive. What does he do? He just goes on to the next attraction & penetrates it. OK,.... well,... I guess I'm on my own to get myself to the surface. I begin my ascent, do my deep & safety stops & surface. He cames up a little bit later (would have been after I would have run out of air). I asked him what he was doing, going inside the attractions & ignoring my call of the dive due to no deco times & my air supply. He replied that he was doing the penetrations to "practice" because he wants to become a technical diver & that he had been teaching himself to do solo & deco diving & thought that my air supply would have been enough. Not long after that day we both basically wound up going to another dive shop. Me because I wanted to take the Dive Con course there & had a little interest in technical diving & him because he wanted the technical training. We both wound up going through the Dive Con course together. He failed the course miserably. He failed the exam twice & got caught cheating on it the second time, was never prepared for the course, gave inaccurate & dangerous information on his classroom presentations, could not complete his swim tests & the list goes on. He even recently told an instructor that he was a Dive Con & could help with a class. Also just after we started going to the second shop I heard it through the grapevine (& it's a pretty accurate grapevine) that he had presented himself as a certified cave diver, trying to get the other cave divers to take him with them. In fact I was there one day when he got caught trying to do that. There were some fireworks! A little time later, I was working at the dive shop. This fellow called up & after a brief conversation, he tells me that he had just completed a rebreather course from yet another instructor, then proceeded to ask me if I wanted to go with him to the quarry & learn how to use a rebreather. I politely said "no", that I had to work that weekend. Unfortunately, from all that I have seen & heard from this fellow, I'm afraid that he one day, will wind up hurting himself or another diver or worse. To be honest I think this guy really doesn't understand fully what he could be getting himself into. I believe that he actually has good intentions, but has completely deluded himself about his abilities & how to reach his goals. Instead of taking the proper training, which is available, he is trying to train himself. Now that hind sight is 20/20, that dive should have never taken place, but I did learn to trust my gut instincts on people & their demeanor, as to how they will or will not follow a dive plan. That is about the only time I've ever had a buddy NOT follow a dive plan, or if it was to be altered, not communicate it with me.