I had an airprox once after flying a microlight off a beach. I filed, as it scared me, and was definitely not my fault.
I put my altitude down as xxxft QNH. The Airprox board phoned me and asked what QNH setting I was using and where I obtained it from.
I explained that I'd set it to read zero on the beach, halfway between the high and low tide marks. The airprox board inspector had clearly never had that answer before, and it confused him!
G