[guipsp]

Your snark aside, clocks are not perfect, and a malfunctioning clock might speed up or slow down.

Also, speeding up the clock is a technique already used in testing enviroments [1].

[1]: https://github.com/majek/fluxcapacitor

[OskarS]

Sorry, I honestly didn't mean to be snarky. It was a genuine question. I couldn't really see the real-case justification for testing a clock that slowed down, sped up, or went backwards. But you're right, malfunctioning clocks would be an example.