Last company where I was hired had the policy to always lock your system when away from it. If you didn't, you would get 'Hoffed' which means that some nice picture of David Hasselhoff in speedo was set on your desktop background. I managed to hoff someone now and then but never got the chance to hoff the only junior dev in our team.

Until one day he was away and didn't lock his system. I quickly wrote a small console app which I installed and added to his user part of the registry. Now he got hoffed automatically everytime he logged on with a random david Hasselhoff picture. Most fun was that he just couldn't figure out how I did that.

On my last day there I told him where to look in the registry ;)