I saw a bumper sticker yesterday that cannot be repeated . . . but this one can:

My mother is a travel agent for guilt trips.

Probably the funniest I ever saw was on a pick-up truck driven by two men wearing cowboy hats (I live in rural Oregon) -- and the bumper sticker said:
If you ain't a Cowboy, you ain't sh*t!

I pointed it out to my husband and asked, "Doesn't that mean that if I remove the double negative a cowboy is a piece of sh*t?" We laughed for days.