Today I saw a signpost on a motorway that stated: "This signpost not in use", which is plainly false.
I was trying to think of other things that exist only to deny that they do. Government leaks maybe? Those pages you find in some manuals that say "This page left intentionally blank" which is another bare faced lie. It got me thinking of other signs you see, like "Part-Time Signals" (it's coming to something when not even a traffic light can get a full-time job) or "Oncoming traffic in the middle of the road" (why not have a sign in the other direction saying "Get on your own side of the road NOW!"). It reminded me of the daft error messages we often present, which lead to a google search for some, which in turn took me to http://www.plinko.net/404/ - the 404 research lab which is worth a few minutes browse. Messages and signs are often ambiguous. See Joels rant on the Microsoft Anti-Spyware messagebox for a recent example but I am sure there are many more to be found.
Anyway back to that sign - I worked out what they should have done. They should have left the first sign completely blank then placed a second next to it with the following message: "That sign not in use -->. This sign fully operational" which would have been much clearer.