I knew this girl who worked at GameStop, and my friends and I knew she was working alone that night. So I bought a mask (some weird plastic thing), waited until GS was empty, and walked in wearing the mask.
Keep in mind I'm 6'3" and 250lbs so I'm kind of intimidating to a 5'6" petite girl.
Anyways, I walk in and stare at her for a second. She says "Hey." I don't respond and walk over to the Wii section and blatantly 'pretend' to be looking at games.
Finally I walk up to the counter where she was. She says "Can I help you?" and I notice she's clutching the cordless phone and turning red in the face. So for a few seconds I just breath heavily. then I remove the mask.
She tried to act like she knew it was a joke, but I felt kind of bad so I didn't call her on her bluff. When my friends came in one of them said "Holy shit, you are RED in the face." and at that point she gave up and admitted that she nearly wet herself and was moments away from running into the backroom and calling 911. It was pretty funny but also she used it as an excuse to yell at her manager and never be left alone in the place at night again.
Keep in mind that I KNEW her, anyone else doing this would have a high probability of getting arrested. That was about a year ago and the only time I've actually filled someone with honest-to-god fright.