When I was about 18 I ran into the back of someone in circumstances that could only be described as comical - even at the time!
On my morning commute to work I had to give way and turn left at a T junction to join a major trunk road. A guy I worked with lived in the next village and because we started at the same time it was quite normal that we'd end up travelling in convoy - he in his car and me on my motorbike.
On the morning in question he was in front as we came to the aforementioned T junction. I saw him check for traffic as he came to the giveway line and move to set off. I double-checked that he was actually definitely going and then looked over my right shoulder for a gap in the traffic on the main road.
All was clear, so I opened it up to follow him out to join the main road. Except that as I looked back he was stationary and about a bike's length in front of me!

I didn't even have time to roll the throttle shut let alone brake before I clattered into his boot in a surprisingly violent collision.
I leapt to my feet split 50/50 between awareness that it was really my own fault for not looking where I was actually going, but also annoyed that he had stopped dead for no reason - unusually there was a long gap in the traffic on the main road.
Turned out his car had died as he put his foot down to join the main road, at exactly the moment I looked away from him. In the end we both had a laugh about it, and no noticable damage was caused (both vehicles were old bangers anyway).
A variation on the classic "roundabout shunt".