When Pee Wee is pulling the insanely long chain from the back of his bike, look under the compartment he keeps it in. You can see the chain coming up from the ground and into the bottom of the compartment.
This mistake is the product of a poor "pan-and-scan" transfer on the home video version of the movie. If you see it on cable now, someone has re-adjusted the "crop" on the picture so this error, as well as the "sign-pulling" error, have been fixed.