YMMV / Dark Heresy

• Game-Breaker: 2nd Edition is a bit fond of this. To the point of being ridiculous even by Warhammer 40,000's standards.
  • The Inquisitor Elite Advance. This is designed to be a reward for an extremely skilled (and most likely, lucky) player. It *must* be awarded by GM fiat. Why? Well, the player becomes an Inquisitor, opening a massive amount of power they can wield now via roleplay. They also can earn talents, most of which give new ways of using Fate points. Examples include preventing the gain of Insanity or Corruption, donating Fate points to another player character, or even being allowed to ask the GM questions about whatever situation they are in, and be given the right answers^note And the GM is instructed to be as helpful as possible. No Literal Genie, no Jackass Genie, just an honest, helpful answer to the current problem. These are perhaps some of the lesser insanely powerful talents. The ones that take the cake though, are Jack of All Trades and Master of All Trades. The first instantly makes most of the Skills list "Known" skills. The second one upgrades all Known skills to "Trained" level. And both of these talents are far cheaper than purchasing training and further tiers in each of the skills.
  • In combat situations in the early game, the Long-las and the Sniper Rifle can get quite crazy. Since they have the Accurate quality they can get up to an extra 2d10 to their damage roll, making them more lethal than Bolters or Plasma weapons on a good roll (not hard with aiming and a good Ballistic Skill), all while drawing less attention, having plentiful ammo, and being relatively easy for starting characters to acquire.
  • First Edition's Ascension supplement also featured ridiculous skills, weapons and especially psychic powers. A decently built character at Ascension rank could slaughter most Space Marines with a little luck and some planning.
  • Speaking of slaughtering Space Marines: Some of the less available weapons from second edition fall neatly under the Game-Breaker status. While none of them does enough damage to insta-kill a Space Marine, there are certain... unbalanced combinations. Weapons with Snare quality require an opponent to make a hard agility check or be considered helpless. Because of this, any damage they receive has double the normal dies. This can be exploited to ridiculous extents - with another strong weapon and a lightning attack talent, it is possible to kill a Daemon Prince in a single round of combat
  • Psykers can completely destroy the game. While the risk of using psychic powers is generally balanced by the risk of rolling doubles, a smart psyker simply saves their fate points to reroll those and can easily go through a multi-hour session without having an accident once. Meanwhile their powers are extremely powerful and versatile, making a well-rounded psyker the answer to any situation ranging from stealth to investigation to combat support to Subtlety manipulation to direct combat. Additionally psykers have the only combination of aptitudes that allows them to take Willpower, the One Stat to Rule Them All of Dark Heresy, at its lowest price since it's also their primary stat. This gets even worse with the splatbooks due to power stacking, so it's entirely possible to have a group of psykers running around punching Space Marines to death with their bare hands.