Mind you I'm still puzzling over a piece of paper I picked up in the street some 30 years ago (and still have somewhere) that is totally blank apart from the words:
"Please destroy this blank page"
Actually someone did come up with a perfectly logical and satisfactory answer to this question a few years ago, a feat which so annoyed me (because it destroyed the nice cosy corner of mystery I had wrapped around the idea) that I have forgotten both the explanation and who gave it to me.
When I was in LA there was someone putting up notices on power poles and fences that just said
"Ignore this Notice"I never worked that one out either.
* I'm sure there is a dead posh philosphical or philological word to describe this but I'm buggered if I know what it is. I leave that sort of thing to people who know what 'philology' actually is and can spell it correctly without using a dictionary to look it up like I just had to.