It wasn't April Fools, but a few months back, a friend of mine called me freaking out because his old high school girlfriend had tracked him down to see how he was doing. She emailed him out of nowhere, nevermind that he now lives across the country and this probably took some pretty good detective work on her part since they haven't had any contact since the mid-90's. This concerned him, since she was a wee bit crazy.
They emailed once or twice, your basic "here's what I'm doing now, nice to hear from you" kind of stuff.
I immediately dropped what I was doing, set up an email account in her name, emailed him and said "Hey Tim, let's get married and have a bunch of babies!"
Twenty minutes later, my work phone rings, and from across the country, my buddy screams "You want to die, don't you!?" He said he'd never been more scared in his life.
My finest hour.