One should also probably keep in mind that a lot of bugs only occur in certain builds of the game.
In these days of mainstream, multi-million dollar titles, developers seem to favor release dates over thorough quality assurance.
With the advent of integrated network play, developers also seem to favor releasing patch after patch (if they even bother) and treat their paying customers as unpaid testers.
The flaw with this approach is that it alienates a sizable chunk of gamers (in this case, gamers who live in a house without a high-speed internet connection).
That, and it's technically illegal in most jurisdictions, anyway.
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.
The sheer number of possible outcomes makes this type of game breaking impossible to fully prevent and even the few games lauded for their stability have an occasional hiccup for which the developers can only suggest reloading a saved game.
Note that the presence of one of these doesn't necessarily make the game itself bad; many programs have been quite entertaining despite horrible bugs.
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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.
Many games that were made after The '90s seem so much easier because of the reduction in such bugs on average.
It was also The Problem with Licensed Games incarnate, since several licensed games actually may not have been as bad as many people say they were...