Here's one I started planning but never got around to actually pulling off. My team had recently moved from Windows to Mac, had a habit of leaving for lunch at the same time and for some reason never, ever locked their screens.

The prank would go like this: I'd grab a copy of a classic Mac emulator, a System 7 disk image and copies of all the Adobe apps circa 1980-whatever, so that they would come back from lunch one day to find all their beefy new Macbook Pros "downgraded" to looking like this, complete with working Photoshop, Illustrator, etc.

I deeply regret not doing this.