Rocket League, every time someone gives any kind of feedback in the chat or disconnects after just one goal. I don't understand the need to be a dick when playing casual and you didn't get the best teammates, or the need to disconnect and look for a new game because 'you were losing' when the opponent scores even one goal. It's not really a single moment but pretty much every single session. Rocket League is a great game, but it's also a toxic one. I haven't played in a while and apparently there might be a fix to the disconnect issue (although it does sound way too strict), but I doubt anything has been done to the toxicity issue.
I'm not sure if this is special for a multiplayer game because I suppose it sounds like it might be pretty common, but I don't play a lot of multiplayer games so I don't really know. It does kind of suck though.