I think, at the simplest, the head- and foot-stones are saying "Don't dig here."
Dark Helmet: Before you die there is something you should know about us, Lone Star.
Lone Starr: What?
Dark Helmet: I am your father's brother's nephew's cousin's former roommate.
Lone Starr: What's that make us?
Dark Helmet: Absolutely nothing! Which is what you are about to become.