Game-Breaker: Space Marines. Not just because they're overpowerednote (and they are — this much was admitted from the beginning, and Game Masters are advised against allowing Space Marines as ordinary characters without somehow balancing the game out), but because they literally break the system with their abilities: thanks to his immense Strength score, a Space Marine can dual-wield lascannons with no penalty, do more damage by throwing his bolter at you than by shooting it, and hurl a grenade across the length of the battlefield with pinpoint accuracy. The Lightning Reflexes talent was also a Game-Breaker, as it fundamentally altered the way gameplay and action assignment worked when used as-written; the game developers attempted to errata the talent, but never decided on a final way to fix it before the game dropped off the radar.
It should be noted that the Dark Magenta fan-written magazine has however written rules that fix these two issues. And in Inquisitor, as all characters are written by hand anyway (with no points values - Ready Reckoner? What Ready Reckoner?), fan material is entirely valid in the game, and even recommended.
Goddamned Bats: Anything that inflicts poison. Past the early levels its no longer direly threatening, but it's extremely hard to get rid of and lasts forever.
fan-written magazine has however written rules that fix these two issues. And in Inquisitor, as all characters are written by hand anyway (with no points values - Ready Reckoner? What Ready Reckoner?), fan material is entirely valid in the game, and even recommended.