Stop me if you've heard this one before...
I was riding an unfamiliar bike for the first time, descending at speed on a wet road, when I was called on to make an emergency stop after a lorry started reversing out of a drive. It took a lot longer to come to a halt than I expected, and the back wheel locking up didn't help. Fortunately I was able to stop with centimetres to spare.
Before carrying on, I inspected the bike to see if I could work out what was the problem. It took me several minutes of head scratching before I realised that the brakes were set up European style - front/left, rear/right.
I'd already done at least 20 miles on the bike, and although I'd noticed the bike behaving slightly oddly under braking before that moment, I hadn't twigged why. That was perhaps not the best moment to find out.