I remember a story on the radio asking the programme to explain
The story teller went on holiday to Cyprus Returning home they opened there door with a Yale (barrel lock) key. Only to realise they used the wrong bunch of keys, bringing home the keys from their rented flat in Cyprus. Their question was how much of a coincidence was that?
The answer was not that much of one. Barrel locks of the Yale kind only had about 1000 differs. (trade term apparently) The "number" is a trade-off between permutations and reproducibility.
The key (ha) to the coincidence was in not surrendering the Cyprus keys on leaving.