Never bake a squash without first making a hole for steam to escape.

This one's unique: never use the Miracle-Gro solution intended for the houseplants to fill the reservoir in the piano's Dampp-chaser. I have a friend who tunes pianos, and she found the strangest things growing in a college's Dampp-chasers. It took a while to figure it out.

(A Dampp-chaser is an appliance which either adds or removes humidity from the interior of a piano, as needed. I have one, but I fill it with very clean water.)

Never plant Grandpa Ott morning glories. Invasive!