I thought of another one:

I use to work on swing shift, there were 3 shifts and there always had to be coverage. The whole crew decided on april 1 to meet in a far away break room and we would all call in sick. The boss was a nervous nellie...he went out and yelled at dayshift that nobody was leaving because all of swingshift was sick, he finally found us and by then he was redfaced and worked up quite a bit.

We tried that one a few years later but by then we had voicemail, before each of our voices saying we were sick was the name of the persons office we were calling from, that boss came knocking on that door we were all busted...we forgot voicemail preceded your message with the person's name you are calling from.