I did the Ride Across Britain in 2016. On the train down to the start in Cornwall, I posted a picture on Facebook. 10 minutes later, I got a response from someone sitting a few carriages further down who had seen my post - it was my best mate from school who I haven't seen for over 20 years. We used to go on long cycling adventures together often when we were teenagers - eg one weekend when we were bored and had nothing better to do, we rode from his house in Ramsgate to Brighton (almost exactly 100 miles), stayed in the youth hostel then rode home the next day. We also went on a cycle camping trip to the alps one summer. Lost touch with him when things got a bit awkward between us a few years later after he started seeing my ex, but all water under the bridge and it was great to catch up with him - and spend some time on the bike with him as well.
Also on the RAB was a chap I'd met just a couple of months earlier on a club ride - we were both members of the club but he doesn't join club rides very often, which is why I'd never met him earlier. Not only do we ride for the same club, we have the same first name, do the same job and my wife has the same first name as his wife. Funnily enough, he was on the same train as well, and in the same carriage, sitting just a few seats away from me, even though we had booked our tickets entirely independently of each other. Despite all these coincidences, we've become good friends.