I destroyed a friend's ability to enjoy Fallout 3/New Vegas/4.

She started with the Fourth and got really invested in the story, and while we were talking about it, I casually said "But we don't know how long you were put back into cryo-stasis after your son was taken--nor do we know what your son is even doing out there in the world. He could be 20 years old, some bandit or raider, and could be one of those dudes you kill when they attack your Settlements. Wouldn't it be crazy if you were in a firefight and this message appears 'You just killed your son'?"

Well, now every time she kills someone in Fallout, she thinks to herself "I just killed my son", so she doesn't play it anymore. I could tell her the spoilers about her son, but then she'd find it pointless to play anyways, at the very least robbing the franchise of the emotional weight it currently has.

I played dark souls 3 with the wikia guide at times so I didn't miss anything.

I fucked everything I could in the Witcher 3.

After I was about done with the nameless king I leveled about 15 levels in.Dark Souls and overpowered myself way too much to beat that fucker.

I believe I called someone an annoying motherfucking piece of crap, which not even his or her mother could love. And who should avoid the media of games at all time for being a camper and deliberately not choosing a map when the rest of us does. Not proud on that one.

During my third playthrough of Resident evil 5 I muted a guy. In the temple in the cave with the solar lazers a bit after the fish boss. Anyway that guy wouldn't shut up like ever. After 15 minutes I was like screw this and put my ear headset off at the end of the level which lasted about 45 minutes he still talked to me. While I ignored him for 30 minutes.

I cheated on Uncharted 4's Crushing and Speed Run Trophies....

There was a game called Choplifter for the Commodore 64. You flew around in a helicopter, shooting tanks and other planes in an attempt to rescue prisoners by blowing open the prison camps and landing on the ground to let the prisoner board the helicopter. Then, you'd return the helicopter to base to save the prisoners. I used to intentionally kill every prisoner but one, pretended that that one prisoner was a foxy babe, and then pretended I banged that chick back at base after I rescued her.

I was once the kid shouting profanity on FPS titles like COD. Then I realized through other matches where I wouldn't do it and just wanted to get some quick games played before I went to sleep just how freaking annoying that was and I stopped.

I co-op played through several Barbie games when I was younger, when my little sister was into them.

Getting 'some' while playing online, course I didn't have to do any of the work ;)

....ok it's not shady or shameful, it was awesome!

When I was young and had even younger kids come over and my mom would insist I let them play Nintendo too, I'd let them play with an unplugged controller, so they thought they were playing but it was actually me, lol.

I was competing with a buddy on COD Ghosts in squads mode and would power cycle the WiiU whenever I lost a match to keep my Win percentage up. I think we ran up into the 70's on overall rank before we both lost interest.

A friend of mine and I were playing Garry's Mod over LAN and we had a few mods installed where we could build more advanced things like Stargate ships and gates. He was hard at work building some ships and I made an entire arsenal of bombs and missiles and I carpet bombed everything into next week. Server crashed a few times too.

Back when I was a kid, there was someone at our school who was stealing kid's Runescape accounts by finding out answers to their recovery questions. He would try to scam people into giving their answers by promising a reward in terms of money and items. I'm not sure what was going through my head but I decided to give him my friend's details instead to find out what would happen and lo and behold his account was stolen! Fortunately most of his stuff was safe in his bank but it was a pretty dick thing to do regardless.

Not a video game but I used to play Yu-gi-oh over the phone sometimes when I couldn't hang out with my friend. I took advantage of the fact that we couldn't see each other so I sometimes went through the deck to draw the cards I needed so I could win. Cheating? No! Heart of the cards!