A college prank, more than an office prank, but a few years ago I was doing a course in Multimedia, no programming aside from some actionscript, so it wasn't a very technical course as such. At the end of my first year, I used a Php script to email a guy in my class, and make it appear to come from our course head, saying something along the lines of "There's a problem with your grades, we suspect plagiarism, please email back to arrange a meeting etc..."

Unfortunately, before I had a chance to tell my friend I spoofed the email, he was already after seeing, and replying it to. Obviously chaos ensued, I got called into a review panel, accused of breaching my course heads email account and whatnot, I had to demo to them what I actually did, and then told they'd review if they would let me continue with the course.

A few days after, i got an email saying they'd overlook the incident and I continued with the course and now have a nice story about a prank that went slightly wrong but worked out fine in the end :)