These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Game Breaker: As a guideline, any rule favoring a specific player is against the spirit of the game. It is, however, quite possible with a bit of ingenuity to rig the game with a well chosen and timed sequence of new rules that do not favor any one player. Introduce "players may play any number of cards, with the top card counting as the played card", but do not reveal it yet. Next time you win, introduce "players may play any card on any other card", and use both rules to win on your next turn, silencing all complaints with "New rule, and previously introduced new rule" (and watch the other players go "wtf?"), then introduce "dealer plays first" when you win that game, causing you to win all other rounds with ease. Don't expect anyone ever to play with you again, however.
Game-Breaking Bug: It's quite easy to make a rule which can break the game and prevent anyone from playing anything at all. When this happens, the players generally take a vote on whether to keep the rule or not.