Marriages are treated as being valid unless/until it’s proven otherwise. that said, some cases are more problematic than others. there are also some cases where the priest stuffs up (for example by forgetting to obtain a dispensation) or there some other reason why things have gone haywire. In those cases if the couple’s consent and understanding of marriage is still ongoing and they still want to be married to each other then often the better (or perhaps less embarrassing / awkward) way to fix it is what’s called a radial sanation (basically a fix behind the scenes) rather than a convalidation. As Fr Z notes though, if the couple reasonably believed that their marriage was valid then their actions weren’t sinful. In any event, as he says, keep calm and call a canonist!