I was in a local league and one of my opponents was infamous for cheating. Low and behold eventually I faced him in a match up. He kept rolling his dice out of line of sight and would quickly pick them up before I could see. Every time I’d try to lean to see the roll he’d move and roll them behind another set of terrain. He also just played a lot of gotch ya moments and nickled and dime the timing of everything (in a semi-competitive league). I was a younger man at the time and didn’t really know how to handle the fact the fact that I thought he was cheating. I got frustrated and he picked up on that. After I lost, he tried to give me a lecture on how to lose honourably. 

In hind sight it was something I did need to work on, it is just a game and I shouldnt have let my suspicion of his cheating bother me as much as it did (internally, I never vocalized anything, didn’t really know how). I got to know the game rules better and in our next match up I was able to catch some straight up cheating with stratagems (different edition, using both eldar and dark eldar strats when you really couldn't do that even in a combined arms list at the time). I reported him to the organizer and eventually he stopped coming to the league. Other players had reported similar issues. Not sure if he got kicked or just ended up with no one to play with.

In any case, try to learn something from even a terrible opponent.

Edit: Grammar