To get a colleague back after he had pranked me, I decided to fill his umbrella up with the contents of a hole punch every day for weeks, I then left the job, but a couple of months later he got in touch to say that the Commander of the Malaysian Navy (was working at Yarrow Ship Builders) had borrowed the umbrella, and had been snowed on in front of other high ranking Naval officers, he apparently saw the funny side though.
While working at YSL, it was common for colleagues to bring in cakes and donuts, but I also used to have a bottle of Daves Insanity sauce, which was used to lace the confectionery.