Loads. Here's a sample.
I was at university in Dundee and doing a summer job in Manchester. I walked out of the office one evening and literally bumped into one of my mates, who lived in Dundee but had come down to see someone in Manchester.
Similar thing happened on 2012, when I was working for Lloyd's Bank in town, walked out the office at lunchtime and bumped into an ex-colleague.
Changed jobs in 1997 and had to work down in our head office in London for a while. They flew us down and on the first flight on Monday, I sat next to an ex-colleague from a former job. On the Friday flight home, I was sat next to another ex-colleague. This was complete chance. The next two weeks, on both the flight down on Monday and the one back on Friday, i was sat next to an ex-colleague.
While in London, working for the same firm, I'd met one of my colleagues for a pint and was heading back to my hotel. I walked down the side of one of the London theatres, turned the corner to walk past the entrance and nearly fell over the Queen and Duke of Edinburgh, who were coming out from the Royal Variety Performance.
September 2006, I was doing a project for Barclays and had a meeting at a building they occupied just off St Peter's Square. My project manager came up from Milton Keynes and we met up in the Starbucks on the corner of Mount Street. The Labour Party conference was on at the Conference Centre over the road and this large group came in, including Bill Clinton and about 20 security men and aides.
Another cracker happened to our kid. He was in Washington DC and waiting to check into his hotel. He got chatting to an American guy in the queue who asked where he was from. When he said Manchester, the guy came out with the stereotypical line "Hey. I have a cousin there. Do you know him?". Turned out the cousin was one of my dad's best friends.
My aunt by marriage had a similar one. Her and my uncle were on holiday in Italy. She was paddling in the sea as was another woman of similar age. They got chatting and this woman (who was Canadian) said "Oh. My ex-colleague and friend is originally from Manchester and has a sister there". Her ex-colleague turned out to be my aunt's sister.
And I've lost count of the number of times I've thought "I must give x a call" and a few minutes later they've rung me.