There absolutely are a few cases. The first that springs to mind is Matilda, daughter of Henry I and claimant of the throne of England during The Anarchy of the Twelfth Century. Matilda had married the Holy Roman Emperor Henry V in 1114 and spent 11 years as Empress of Germany. While she was away, her brother William had been killed in a shipwreck in 1120, leaving England without a male heir, and so in 1125, Henry V dies and Matilda returns to her father in England where she is nominated as his heir. Contingent on this is an alliance between England and the French County of Anjou, which would protect the borders of Henry I's lands in Normandy. In order to seal this alliance, Matilda is coerced into marrying Geoffrey of Anjou, which she resists until 1128. At this point, Matilda is a 26 year old widow, and Geoffrey is only a 15 year old boy, although the two do have a son - the future Henry II - in 1133.

Yes, it's true that large differences in age weren't as common as people think, but they did certainly happen. It was more common for a husband to be older than his wife for a number of reasons (the pressure to have children, the fact that canon law allowed women to marry at a younger age than men, the need to provide for a family, and more). But equally, there are instances where the wife was older than the husband. Usually these wouldn't be major differences in age, but there were always outliers from time to time.

One particularly well-known example is the so-called 'diabolical marriage' between Catherine Neville, Duchess of Norfolk and John Woodville, a brother of Elizabeth Woodville, in 1465. He was 19 and she was 65. As you might have guessed from the nickname given to their union, contemporaries were not impressed. Fun fact: despite being nearly fifty years older than John, Catherine outlived him.