At first I thought it was a joke, but after reading other warnings such as: "Hot Cheese May Burn The Inside of Your Mouth" and " Plastic Pizza Guard is Not Edible", I'm thinking that they're serious.
Jesus hot-pants-wearing Christ, How fucking stupid have people gotten that we need this warning printed on the bottom of pizza boxes? I'd wager that if you're stupid enough to carry around your pizza upside-down, you wouldn't be able to read the damn warning anyhow.
Viva la de-evolution.
Props go out to our own pizza box holdin' Italian stereotype, Mr. Joe, for his modeling services.