Another Blizzard game, Diablo 2, actually had a random number generator bug that went unnoticed for years.
Essentially, any random chance in the game of the form 1 to n could only ever be 1 to n-1, I think with n-1 being twice as likely.
This went unnoticed since most random rolls were over fairly large ranges and it didn't seem to hurt much, however it did explain why one particular subtype of loot literally never dropped.
And you know, it is always possible in those situations that there is a broken pseudo-random number generator involved. But of course, that's really hard to tell too because real randomness is so unrandom seeming.