An old punster was locked in a closet until he could come up with a pun. As soon as the door was latched a muffled voice came form the other side of the closet door, "Oh pun the door".

Okay, nevermind that one. Here's a couple of real life one's from right here in Harpers Ferry:

PEED PLUMBING

And for the tubers who cool off in summer tubing down the Potomac:

BUTT'S TUBES