Chaton, My wife who wears hers on the loose side has this problem often. On the other hand mine are on the tight side and I've never had it happen.This has happened to me too. When the bracelets are worn loose and they are just at the right angle, this can happen.
It would not happen on a tighter fit. However, I don’t like the tight fit.
Does this just happen to your small and classic (that’s the combo I have & it’s happened)?
Has this happened with your two classics before? If I remember correctly, I think it also happens with the classics. I recall reading that a user, who has two classics, stated her two bracelets switch places as well.