Several times during the series, Mr. Gold is seen wearing a wedding ring. This is because Robert Carlyle has forgotten to remove his own ring; he has even tweeted about it.
Throughout season 4, Regina is seen to be wearing a wedding ring. While Regina is not married, Lana Parrilla was wed to her husband just before filming for season 4 began and probably forgot to take her ring off when shooting.
The flashbacks shown in season three, particularly of Prince Charming show him with his current hairstyle not the hairstyle he had in season one/the Enchanted Forest, thus, making him look older/different than he should do at that point in history.
Despite being in a fantasy Medieval setting while in the Enchanted Forrest , costumes have modern zippers (not invented til 1893/1913) , best example is Regina's pilot outfit (on display in Disneyland California Adventures )