(childish) if you read this you're stupid (customary scribbling on school desks etc.)

to put the cart before the horse

A saying of memento mori, often written on cemetery walls and the like; literally: what you are, we were; what we are, you will be

(idiomatic) to throw the baby out with the bathwater

(idiomatic) to pull the wool over someone's eyes [+dative]

(idiomatic) a disaster happens or becomes inevitable; it is too late for precautions that should have been taken earlier

(idiomatic) to have one's heart in the right place

a burnt child dreads the fire; once bitten, twice shy

(idiomatic) to not hold a candle to

(idiomatic) to tip the scales