I've had heaps of these...
On an extended holiday in 2004-2005 I ran into the same couple once in Rarotonga, twice in New Zealand, and once in Kathmandu.

Then in 2008, my girlfriend and I met the same couple in Damascus and Manang, Nepal.

But the all time best one was when my girlfriend and I ran into a good friend from Toronto entirely by chance in Khulna, Bangladesh.

At that point we'd been in Bangladesh for a bit over a week and had seen only two other obvious foreginers.

I was sitting in a hotel lobby and to my utter astonishment, in walked my friend Susan. Though we'd been good friends in Canada, I'd been living in New Zealand and we hadn't kept in regular contact. I knew she'd been volunteering in India for a few months, but I'd assumed she'd gone back to Canada after finishing.

Instead she took a trip to Bangladesh to get an Indian tourist visa, and ended up running into us. We spent the next couple of weeks travelling together, and had a great time.