Any funny, nostalgic, or interesting anecdote related to that?
Here's mine:
My dad and I had an ongoing rivalry during the 90s (mostly when I was around 8-12 years old).
Because I was younger, I had better reflexes and a few other gaming advantages.
My dad compensated with intense old-school cheating. For instance, in:
Command & Conquer: Red Alert. My dad and I played LAN with two computers in two adjacent rooms linked by small stairs with no walls. Inititally, I was unbeatable, but suddenly he turned the tables and won game after game. I was clueless. I just couldn't get why he started winning. I even double-checked the instruction manual to see that there wasn't some spy construction in the game that I had not heard of. But years later my dad admitted: He had put up a set of mirrors in the apartment so that he could see my computer screen from where he was sitting and spy on my base through the mirrors. Apparently one day I had asked him why there was a mirror standing next to one of the chairs and he then removed all the mirrors because he thought I was on to him.
Sega Rally. On the Saturn. Inititally, I was constantly beating my dad, gaining time on him on almost every turn on the desert stage. He practiced a whole lot though. I typically woke up early on Saturday mornings to hear a familar, lovely sound from the living room (VROOM, VROOM, VROOM, SEGA RALLY CHAMPIONSHIP). Despite this practicing he could not beat me. Until one day, when he suddenly raced pass me on the long, straight parts of the stage. I just couldn't get it. I guessed that he took the turns in some unusual, novel way that gave him extra speed on the long runs. I tried to change my driving style accordingly, but he just kept on winning and laughed and smiled quoting his race times and mine... Weeks later I discovered that you could increase vehicle speed in the settings.

He cheated also on cards, or any game for that matter. Ironically, he always accused me of cheating, but I think it made me the opposite. Thanks to him, I'm an honest man.

So what's yours?
Anecdotes in which you are the parent together with your kid are also welcomed.