Broken cranks - my first ever one was in a three-up. Luckily I was on the back at the time. Bashed my knee a bit, and we all got cold getting back to the start, but at least we didn't have a pile up.
Not strictly mechanical, but last night the trains out of KX were a mess. The whole station was running for anything moving. No chance of getting on one, even less with my folder to fit in as well.
So, I gave up, went to St Pancras, caught a train to Harpenden, and rode home 14 miles from there. Unexpected ride, so hadn't made sure my main front light was well charged. It gave out 100 metres from home
Nice ride though on quiet, country roads in the dark (sorry, this bit should be in "Have you been out today").