"Someone very clever [...] must have made the clock for the Patrician's waiting room. It went tick-tock like any other clock. But somehow, and against all usual horological practice the tick and the tock were irregular. Tick tock tick... and then the merest fraction of a second longer before ... tock tick tock... and then a tick a fraction of a second earlier than the mind's ear was now prepared for. The effect was enough, after ten minutes, to reduce the thinking processes of even the best-prepared to a sort of porridge. The Patrician must have paid the clockmaker quite highly."
Terry Pratchett, Feet of Clay
"The clock in Lord Vetinari’s anteroom didn’t tick right. Sometimes the tick was just a fraction late, sometimes the tock was early. Occasionally, one or the other didn’t happen at all. This wasn’t really noticeable until you’d been in there for five minutes, by which time small but significant parts of the brain were going crazy."
Terry Pratchett, Going Postal