One of my colleagues likes to smother phones in hand cream, so when somebody picks up their phone, it gets all over their face & hair. I always check my phone, so she's not got me yet!

Once, I popped out during lunch and when I got back, EVERYTHING on my desk was gone. I was so confused! One of my colleagues had moved all my stuff from my desk onto an empty desk on the other side of the office.

Also when somebody leaves their computer unlocked, it's commonplace to send emails from their computer. From offering to get the drinks in for the team, or declaring their undying love to someone.