Unlocked PCs
Darren wrote about leaving his machine unlocked when he went away to lunch.
Here's a way to really annoy someone... saw it done once.
You take a screenshot, using the PrintScreen button. Save it to an image, and set it to the background. Then you lock the machine, and move the login box to the very side, where it's hardly visible.
Much better than being malicious of course, and they tend to get the point...