Simple. When the original poster tries a new pair, they assume it may not work. If they happen to feel pain they mentally mark the glasses as “one of the bad on

Simple. When the original poster tries a new pair, they assume it may not work. If they happen to feel pain they mentally mark the glasses as “one of the bad ones”. From then on the placebo effect won't apply to that pair because they don’t believe it will help. Humans are great at finding patterns in randomness like this; this is exactly why superstitions are so prevalent.