I pranked someone (probably not on April Fools) at an office job I had in High School decades ago. I had a summer job digitizing documents.

I discovered that I could access the Startup folder on other employee's machines on the network via Windows Explorer. I put a script in one of my very rule-following co-worker's folder that was something like: dir dir dir dir (x100) echo All files have been deleted.

I watched them from around the corner when they booted up, saw the flood of file names flash across the screen, and flipped out when they read the message at the bottom. They reached for phone immediately to call the IT admin and I rushed out from around the corner explaining the joke. Never got in trouble. Good times.