The issue was a ring used on the lower clevis of a head stay on a Pearson 36. The ring was undersized and was pulling back through the hole due to wear.
Not certain I'm reading you right, but it sounds like you're saying the ringding was being pulled through the clevis pin hole? If that's the case, I'd argue the clevis pin diameter was too small, as well.
This thread does
remind me of an incident this spring. We were stepping the mast when one of our lower shroud's clevis pins fell out of its fork, bounced off the deck and *plonk* - into the drink it went. They all had had ringdings on them. So that's another case I've seen personally of ringdings working themselves off hardware. Related: Twice
I've had a split ring on my keys manage to work itself off.
I think next time we're out at the boat I'm going to throw a bit of tape around all the ringdings.