That was the question asked over on Clusterflock, with the following anecdote offered as a starter:
I increased [my boss's] sugar intake by one spoon at a time and generally left a couple of days in between changes. I kept going until I was bored. I guess part of me wanted her to notice, but she never did. Not even the amount of sugar we were getting through. Anyway, I eventually got her up to 23 spoons of sugar in a cup of tea -- yeah, 23! She never said a word and always finished the cup.
Reminds me of some of the stuff that Jim does to Dwight on The Office. At my first job out of college, some coworkers of mine and I found a Mac OS extension that decreased the size of the screen by a pixel or two each time the computer booted. I don't think we ever installed it on anyone's computer; just imagining the reaction was good enough.