The best I saw was advised to me by a female friend - seen on toilet cubicle door at Keele University, and proving that a reply is often wittier than the original, which was -
There's a village in Central Scotland called Crook of Devon, so called because it sits on the crook of the river Devon. On the signpost leading into the village someone had added under the village name "Twinned with the thief of Baghdad".
Some others:
Richard the Lionheart is not dead. He's alive and well and asking Christian Barnard for his money back.
Jack the Ripper is not dead. He's alive and well and doing my laundry.
Dyslexics Rule - KO!
Or for the thinkers among you - The urge to destroy is creative.
The best I saw on a condom machine was .were it said
Electronically tested, some bright spark drew an arrow pointing downwards followed by the words.
SO WAS THE TITANIC!!!!
A sign at the side of the road boasting the prowess of speed cameras said, police have caught 562 speeding motorists already this year, underneath was written..................... and not one bloody burglar!!