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: The Paladin class as a whole. In Quest for Glory III, once the Paladin earns the "Heal" ability he's granted the Magic skill. By Quest for Glory V the Paladin can learn every spell in the series, except for the handful of that specifically require being the Wizard class to learn (most of which are plot-specific). The Paladin is therefore not only a hybrid Fighter/Wizard, but he also carries the best sword in the game, and has his own class abilities on top of that!
Unwinnable by Insanity: Attacking Tanya or Toby in Quest for Glory IV doesn't immediately kill you; Toby will just force you out of the room. But it makes the game unwinnable, as Tanya will charm and bite you if you go back into her room after that.