Tom Clancy's Splinter Cell
In the CIA HQ, once you pass the Media Room where the agents are reviewing a sound bite, there is a room on the right where you find a computer. If you turn off the lights in that room, the un-lit flourescent bulbs turn into a note for developers that was never changed for the final version of the game: giant red blocks that say "Change Me!" on them.
Contributed By: paulwind.