Illegal Dumping Prohibited
This one’s a head-scratcher, in that it’s a bit of a truism that illegal dumping is prohibited. By definition, it is prohibited everywhere.
I assume the intention is to acknowledge that some dumping might be permitted in this location — so you might see a bag of trash here — but that doesn’t imply that you, stranger, can dump your trash here.
(Thanks to Sheldon and Thea for this picture, which was taken in Halifax, Nova Scotia.)