This thread is too much fun!
On a hike up to a cave I saw a sign that said, "Running can be hazardous to your health." I took a picture to turn in at gym because I hated running.
At Bryce Canyon in Utah there was a sign that said to find shade and wait for help if you get lost. Not so bad, but the picture they had on the sign showed miles of desert and a sad tiny tree that provided next to no shade.
While shopping at the toy store a few weeks ago we I saw a sign that said "Save $0!" Upon closer inspection, it turns out that the sale price was $44.99 and the original price was $39.99. It made me laugh as is, but part of me wishes it said "Save -$5.00!"
Last one I can think of right now is the name of a town in South Dakota. It's called Pukwana and the smell around it was horrible! I don't know that it should be pronounced puke-wanna, but it sure seemed appropriate.