Some Fridays (once a month?) were casual dress days where you could wear jeans instead of slacks (this was the distant past, when most professional workplaces still had real dress codes). This was an IT/Eng-wide thing, so we'd get an email reminder about this from an admin person in the department.
One time, I thought it would be funny to send my own email announcing pants-less Friday. So I took a copy of the email this admin sent and adjusted it accordingly. I did of course specify that you still had to wear underwear. I'm not a monster. Because I had programmer privileges in Notes, I was able to forge the sender so that it appeared to come from the department admin person, not me.
I _meant_ to send it to the small email group for just the other tech support folks (around 15 people or so). But I accidentally (?) sent it to all of IT/Eng, around 200-300 people, IIRC. Oops.
Needless to say, my boss's phone started ringing off the hook. I immediately went over to tell him what I'd done. He wasn't pleased, but I didn't get fired. I did have to write an apology email.
Of course, many folks in the department later told me it was the funniest thing they'd ever seen happen.
Soon after, I moved to programming at a different company. I think this was a good thing for many reasons, but one reason is that it was more challenging, so I wasn't bored with time on my hands to do stupid things like send prank emails to my coworkers.