I had typed up an e-mail via manual commands to the SMTP host to make it look like an e-mail from one of our clients, and sent it to my boss. In the email, I put a reference to a deliverable (a software component; twas a software project) and a list of errors that the client had found. In the deliverable, I put an "easter egg" that had a section that read, "April Fools!"
I felt bad at first, because I imagine her stress level went quite high upon seeing the errors. She felt better after seeing the message though
