OK, so here's a weird one...
Several months back, I bought myself a Lens Flipper. It seems to be a really useful thing to have! More recently, I bought a couple of Sigma lenses. Last night, I discovered something baffling: Even though both the Sigma lenses fit perfectly on my Nikon camera body, neither of them will mount properly on the Nikon-fit lens flipper. One lens will mount on one end of the flipper but not the other, the second lens point-blank won't mount onto either end [at least, not without applying far more force than you should apply to a delicate piece of precision engineering!]
I don't understand how this is physically possible. The mount on the lens flipper and the camera are identical. All my Nikon lenses mount effortlessly on both. The Sigma lenses mount perfectly on the camera body... but somehow not on the lens flipper. What's going on??