Years ago, I was a tutor at a residential college at the University of Sydney, where I had a small apartment. One summer afternoon I was having a nap, when the phone beside the bed rang. On the other end were two college students from Pittsburgh in the USA. They were having a BBQ, were a bit drunk, and decided to ring random international phone numbers. We had a chat, exchanged addresses, and sent each other postcards.
A couple of years later a friend who was in the middle of his PhD research went to Pittsburgh. He was standing in the middle of the foyer of the university library wondering where to find the catalogue (all of this is before the internet), when a couple of young men approached him. "You look lost", they said. "Can we help you?". He explained that he was visiting and didn't know the library layout. They commented on his accent and asked where he was from. "Australia", he replies.
"We only know one person in Australia", they said. "She lives in Sydney. Perhaps you know her?". "Sydney's a big city", he says, "it's very unlikely.