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.
There are some people, my partner being one of them, who run their rigs so loose that they actually have slack.
He claims that he is able to fine tune the rig while racing. I can't argue with his results because he usually wins. However, it's not a tactic that I would recommend to any of my customers.
In my opinion, your standing rigging should never, under normal conditions, be allowed to slacken to to point that your clevis pins move around freely.
There is a certain amount of movement on a headstay, but with the use of toggles for articulation, your clevis pins should never, under normal conditions, move around in it's hole.
Having a key ring work it's way off is really not the same thing as your standing rigging.
Still, as I showed in the picture, I use circular rings to lock the turnbuckle and a straight cotter pin for the clevis.