Game code is an set of complicated instruction (that spans to thousands of branchig if this than that kinda situation).
Due to the huge amount of branching of possibilities, there will be a set of choices that might lead to unexpected outcome that they did not handled.
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Game also has to operate within numerous amount of different environment (that is your pc).
Some of this environment is simply better for the game compared to the others (due to them being more tested and its hard to test on all known combination of used hardware that is recommended for the game)
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