World of Warcraft's Mythic+ system, wherein players can time-trial endgame dungeons for a chance at loot and bragging rights, has been a bit of a curse and a blessing for the game since it was introduced.
It's been a blessing because, well, it's become a massive scene—letting small-scale groups get all sweaty without having to commit time to a raid.
It's been a curse because, well, this is World of Warcraft, and it's difficult for anyone to get along. In case you're uninitiated, here's the gist—in order to access a Mythic+ dungeon, you need to grab a key, which directs you to a specific dungeon with specific modifiers.
Once you've grouped up and activated the key inside, it's locked in. You either beat the timer and get your phat loot, or you don't, and suffer the consequences.
This means that anyone in your party has the power to simply 'brick' the key by ding-dong-ditching. Open a door for griefing to happen in WoW, and players will take it.