how uneducated people spell fool-proof.
despite any arguments for the validity of the word "fullproof" the correct spelling literally means 'even a fool couldn't get this wrong'.
clearly the spelling of the word fool-proof is not fool-proof.
(on Facebook)
John: Your plan to create a seagull army is ridiculous...
Sarah: No it's not! It's fullproof!
John: It's foolproof... and that's fullproof evidence that it's not, bitch.