Transpose unlimited INT or CON.
Druid 2 transformation channel crash with rollback (Items overenchant +20++ and reopening world boss and bosses cubes until you get good drops, controling world bosses respawns times by crashing the same channel multiple times.)
Thauma Reverse being able to reverse skills on world bosses should only work on pvp.
Monk plus Cryo ice wall and energy blast combo crashing players games winning GvGs and Worldbosses.
10 people party abusing invites on load screens and accepting at the same time for grind or world bosses.
Scout c3 scan giving hats for other people BUT counting ice walls, fire balls, servants as players and giving a lot of hats to the same player.
i think i listed all the game breaking ones here.