Most pointless placard?
When I learned to fly in a yellow Tiger Moth, there was a stern warning on a little plate bside the fuel filler cap to the effect 'maximum for aerobatics 18 gallons'. The tank held 19, as I remember.
As our CFI remarked, he had yet to meet anyone who could fire up a Tiger, taxy out,do checks,negotiate with ATC, take off and reach a survivable height for aeros without burning a gallon!
Any other illogical placards spotted?