I recall shooting a BSS for skeet at night for years. Once in a while, the front bead (a light gathering plastic tube type) would be noted as missing part way through a night of shooting. It never seemed to matter on my scores.

But I believe that is because of transference to other references thru repetition. You shoot 10,000 times while focusing on the target, but your mind sees the barrels, your hand and forend, rib, etc., over and over again. Not just the rib or bead.