This guy explains how he changed the text on some light-up roadside signs which were still in operation long after the construction crew had finished its work.
This was the first time I had attempted a prank like this, so I expected the control box to be locked, and the programming functions password-protected. I was wrong. First of all, the control cabinet had no lock. Swinging open its door, I found a deliciously inviting handheld keypad, then took a wild guess and pushed a button labeled STOP. The display on the control box flashed ENTER PASSWORD. I was about to give up in disgust when I noticed that someone had written the password in large Sharpie lettering above the box.Link (via Neatorama)