And yes, do beware those pesky jumping pebbles.
My first full-time job was in an amusement arcade in Blackpool. After lots of problems caused by misbehaving pennies in the machines, the owner had obtained a brand-new state-of-the-art penny-pusher, with all the flaws of previous models designed out. You may know that when you nudge the machine to try to get that huge pile of coins that has been balancing on the edge since you started pumping money into it that a bell rings, and a solenoid closes the outlet chute, causing all that money, which always will at that point fall off, to drop into the owner's box. On one occasion, about three weeks in, the bell was accompanied by a burning smell. A falling penny had landed vertically into the mechanism at the very moment that the solenoid was activated, and jammed it.
I gazed in wonder at the sight when we opened it. The boss shrugged his shoulders and said it doesn't matter what they say or do, you can't beat the odds, no matter how big they are. So I agree, beware those pesky jumping pebbles. It isn't just that law that states that if something can possibly go wrong, it will. That law also applies where nothing could possibly go wrong.
(The boss was in truth upset because it was the biggest earning machine in the place, and was 50% out of order until the new solenoid arrived the next day.)