The growing prevalence of Wreaking Havok (especially in the context of facilitating emergent gameplay) can often cause essential game entities to be launched or pushed into places outside the player's reach or destroyed through unexpected methods.
In today's market, where even console games can be patched, it's incredibly rare to have a game-killing glitch maintain itself for very long.
The very worst of these can cause a game to be Unwinnable by Mistake no matter what the player does (except, possibly through a counteracting Good Bad Bug).
Bugs that always happen at the same point of an Endless Game are known as Kill Screens.
Not necessarily the same as a Game Breaker that results from a programming bug; those are typically Good Bad Bugs, which are harmless, but examples of ones that use Game-Breaking Bugs exist; read on...
The dark side of Good Bad Bugs (though not necessarily mutually exclusive) and a Griefer's favorite variety, Game Breaking Bugs are severe bugs that cripple your ability to play the game involved. Game-Breaking Bugs were more prevalent in the earlier days of gaming.