Foolproof-Fullproof | Commonly Confused Words

Foolproof

/ˈfuːlpruːf/

Designed to work correctly under all possible conditions, even when used incorrectly

Extremely effective or failsafe

Foolproof starts with an F just like fail - it suggests that something is designed so it can't fail.
Fullproof is not a correct term in standard English, remember full means complete, so you might confuse it with something that is completely right or certain.
People often think fullproof means completely secure, but the correct term is foolproof.
If you see fullproof, think its a full mistake because its incorrect.
Foolproof with fool implies even a fool could use it without error.