Surfing wax

I always assumed that surfing wax (as in the infamous Mr. Zog's Sex Wax) was like wax for skis-- it's there to make the bottom of the board slicker and reduce friction.

Instead, it's actually the exact opposite.  You apply the wax to the top of the surfboard, and the waxed surface has better grip and traction for your feet.  Apparently the material that modern surfboards are made of is well and sufficiently slick on its own in the water.