i've been thinking about this for weeks and no amount of googling has helped.
i suddenly remembered a plot from something i watched years ago. unfortunately, no identifiable details are clear to me (character appearance, location, era, et cetera) and i can't even remember if it was a film or television program.
one character finds a piece of jewellery (a necklace? a ring? something else?) made of metal. this item is the exact same shape as their friends' birth mark. it's a really weird coincidence. the resemblance is exact, and it's a very distinct shape. they buy it for this reason. the two characters are amused and weirded out by this very strange coincidence.
later, the owner of that item travels back in time, and meets their birthmark'd friends' parents. said friend is also present, as a baby.
somehow, the baby gets burned by the item, meaning their birth mark was actually a scar from this burn. a burn from an item their friend wouldn't have even owned if that burn hadn't happened in the first place. a paradox.
i don't think this was The Plot of whatever it was, just a sort of fun paradox thrown in at the beginning and end because time travel writers cannot resist paradoxes.
i have been trying for ages and i CANNOT find whatever this was. did i make it up? it feels like such an obvious plot, like it's probably been done before, yet strangely i can't find any examples of it, not even "same plot, wrong film" or something.
does anyone have any idea what i'm talking about?