I have caught people cheating on a table I was playing twice in the many years of playing. Both times I reported it, and the
poker site not only sided with me, they had also gave back the starting buy in for everyone at the table.
What happened was they were "chip dumping". This is where one player usually the stronger player. Loses the hand on purpose with a small bid into a big pot by the other player. It happens sometimes that some players don't realize that folding a hand with a big pot, by say a bid of a 40 chip bid when the pot size is over 2000 chips is not a good idea.
With that being said, when it happens consistently you have to think that collusion is really what is happening. The one player keeping the other player in a tournament in my cases.
It could have happened more, but it doesn't make sense to think that everyone is cheating. Some people are just bad players and rely mostly on luck. You have to take the good with the bad though. You can't rely on just plain dumb luck and win consistently.
I did just yesterday think to myself that man, this guy has to know what cards are coming. lol Most likely not though I am sure.