I bought some sheetrock at the local Home Depot and had to remove my bed extender to slide the sheetrock into the bed. I put the bed extender on the ground below the tailgate. Well the weight of the sheetrock lowered the truck enough to press the tailgate down on to the extender.....doh! No damage just a scuff on the plastic ends of the extender. I had to lift the tailgate to rock the extender out from under the truck.

 

Learned about the tailgate/bumper on my last truck with a set of keys. Small dent, but like you said, you know it's there.

 

I set a wad of keys on mine...

 

I have a bike rack that connects to the trailer hitch. I just finished taking it off so I could get stuff out of the back I dropped the tailgate and looked down to see the socket, still attached to the ratchet, sticking up beside the tailgate. I cringed and slowly raised the gate. Would you believe the handle was *just* thin enough not to make contact with the body. Whew! I would have been torqued!