Correctness of 'whole' program execution depends on order of instructions executed.
If hacker can hack processor clocks (for example in distributed system), he can introduce subtle imbalances in a system, making it behave nondeterministically...
Such errors can or can't cause program to crash or provide wrong results.
There is example of computer program that worked well until processor was upgraded, speed changed, and 'whole' program stopped working because of bad synchronization.
Such attack is more dangerous than it seems, and often more stupid than Nuclear Bomb. But it should be learned and mastered to counter such.