Had the exact opposite happen once — playing Risk with a group of friends, one guy ends up completely dominating the board. Almost everyone is out, just him and one other person left.
Everyone wants to either start a new game or go to a pub, but he still hasn’t won. We ask him several times if he’s accomplished his goal yet; he hasn’t.
He drags the game out by an additional 3 hours, and finally wins. Turns out he had won some 4 hours earlier, and “forgot” or “misunderstood” his objective. In reality, assholes just wanted to try to take over the whole world, and thought we wouldn’t insist on seeing the objective card afterwards.
We banned him from participating in any and all games. For life. This was some 25 years ago. I haven’t seen him for 10+ years, but to the best of my knowledge, nobody from that particular friend group (around 12 people) ever let him participate in any games again.