When I went punting in the rain in Cambridge, we passed this little blue door leading from one of the university colleges (unfortunately, I can’t remember which) and our guide stopped to share an amusing tale with us.
Apparently, for a few years, the running prank for freshers’ week was to change all the fire exit signs to point toward these little blue doors. Then in the middle of the night, the senior students would pile onto the bridge (which you can’t see here, but it’s just ahead) and wait as one of them pulled the fire alarm. Moments later, they’d watch and laugh as the half-asleep freshers ran out and fell into the cold water. It must have been hilarious to watch, but quite the experience for the freshers.