Mr. & Mrs. Smith (1941)

"Lombard and Montgomery Get Hitched As They're Getting Unhitched !"

Happily married for three years, Ann and David Smith live in New York. One morning Ann asks David if he had to do it over again, would he marry her? To her shock, he answers, "No". Later that day, they separately discover that, due to a legal complication, they are not legally married.