What are some harmless practical jokes?

The Blue Screen of Death screen saver, available from Microsoft, is sure to briefly terrify a coworker, without actually doing any damage to her files. If you just want to cause confusion, hide air fresheners all over her office.

In order to preserve feelings, stick with pranks that are confusing or odd, rather than those that rely solely on scaring the person being pranked. Try changing clothes throughout the day without mentioning it to anyone. Put up a sign that says your office is for sale. Use some photo-editing skills to create an eerie head in a jar and leave it in the refrigerator. Start the night before to put office supplies in Jello.