Queen Elizabeth II may have been the British monarch, but even she was not immune to accidental faux pas.

This week, royal fans and experts looked back at Prince William and Princess Kate Middleton's 2011 wedding at London's Westminster Abbey as they marked 13 years of marriage Monday.

Speaking to Hello!'s "A Right Royal" podcast, etiquette expert Alexandra Messervy recalled how Queen Elizabeth, who passed away at age 96 in 2022, broke royal protocol at her grandson's wedding when she got out of her car on the wrong side.

According to Messervy, royal protocol dictates that women should exit the car on the curbside instead of the street side.

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However, on the day of Prince William and Middleton's wedding, Queen Elizabeth and Prince Philip's car pulled up on the wrong side of the street in Dean's Yard, forcing the monarch to get up and exit the vehicle through the door nearest to her husband, as seen in a video shared by The Royal Family Channel.

"The queen got out on the wrong side and there was a bit of a fluster," the etiquette expert told the podcast. "The correct protocol for her was to be on the curbside, and she was on the reverse side."

She noted that the guard who opened the car door where Queen Elizabeth II had originally been seated "appeared flustered" when she got out on the other side.

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Messervy explained that the tradition stemmed from "the days of carriages and horses."

According to the expert, the male members of the royal family were seated on the pavement side, and women were on the opposite side to "avoid the mud."

But Messervy suggested that there could be other reasons why Queen Elizabeth exited the car on the opposite side of where she had been seated, such as a "sore leg."

Britain's Queen Elizabeth II
Britain's Queen Elizabeth II speaks with staff during a visit to the Defence Science and Technology Laboratory (Dstl) at Porton Down science park on October 15, 2020 near Salisbury, England.
(Photo : Getty Images/Ben Stansall - WPA Pool)

On Monday, the Prince and Princess of Wales marked their 13th wedding anniversary with a previously unseen photo from their nuptials.

The black-and-white photo posted on their official social media showed Prince William and Middleton smiling as they posed together for a portrait.

Princess Kate looked radiant in her Alexander McQueen wedding gown and Queen Elizabeth's Cartier Halo Tiara as she held her lily of the valley bouquet.

The couple captioned the sweet snap: "13 years ago today!"

A post shared by instagram

This year's anniversary was a little different for Prince William and Middleton as she continues to undergo preventative chemotherapy.

The princess announced late last month that she was diagnosed with an undisclosed form of cancer after undergoing abdominal surgery in January.

Royal expert Christopher Andersen suggested that Middleton's cancer battle was likely the reason why the royal couple chose to share a throwback photo instead of a current one like they did last year for their 12th wedding anniversary.

"It is interesting that they have apparently chosen not to release a current photo of themselves," the royal biographer told Us Weekly. "Perhaps, given the fact that the world knows what they're going through at the moment, smiling for the cameras might seem forced."