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Surely you've had a few opponents that have cheated. I am certain I had a couple of opponents who used some chess engine against me. I checked the games with Rybka AFTER PLAYING THEM, and I saw they made the first or second best move all the time. And I'm talking about 1600 players. However, players who cheat like this are not the ones who really concern me. If you play engine moves all the time you will be detected by the system. And even if you're not detected, what you're doing will become really obvious to your opponents. So once they know you, they'll stop playing against you.
The ones that concern me are players who play their own moves most of the time but who are constantly checking with an engine to assess the position. Whenever the engine sees a clear blunder (-2 or more), that's when they play the recommended move to punish it. So the system doesn't detect them, they cannot be blamed just for seeing a blunder, and there's no obvious way for their opponents to know they're cheating.
How do I know this kind of player exists? First, because I'm talking about an easy and uncreative method and sick people will always take advantage of such things. Second, because I know one of these players. He's a very special case where you can tell. I played against him constantly, and I used to always beat him (ALWAYS). He's 150 points lower than me, and he's still bad for his rating. One day I posted on his profile that if he wanted to improve he shouldn't make so many stupid queen moves in the opening (I know I shouldn't have, but sometimes I'm just obnoxious). He replied that he had a special style and that he could beat anybody and he would certainly beat me every time we played. And there you go, the next 3 times we play he crushes me after playing like a total beginner in the opening. I checked our games with Rybka later and she didn't suggest all (not even most) of his of moves. However, she answered my blunders exactly as he did. They were not evident blunders. They were almost impossible to see, for someone of his level.
It's not that I want this particular person to suffer the consequences (I'm not even posting his name, since I have no absolute proof anyway). I can just stop playing against him. But since then I've been wondering how many people do this. And it really concerns me because, for the reasons I explained, I don't know what online chess communities in general could do to detect and ban these people.
Does someone have any ideas? I'm really interested.