Mustard
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Here's my new "you left your computer unlocked" image
My favorite was to screenshot the desktop, set that as the background, then move everything on it to a different folder.
You can just disable desktop icons in windows. It's both less destructive and more infuriating.
Flip the image upside down and then flip the screen upside down as well so it looks normal. Except for the cursor, which will be upside down and moving in the wrong direction.
In the good old days we would just do that prank by taking the ball out of the mouse and putting in again backwards.
Just be careful not to tip it over so that you don't spill the pointer fluid.
We used to shout in the company chat about how much "i" love the company.
Very bad spanish sentences were fun too.
I think ours were about wanting hugs from pink butterflies.