They committed arguably the biggest wedding-guest faux pas there is.

After Meghan Markle and Prince Harry were pronounced husband and wife on May 19, 2018, they joined 200 guests in celebrating their new marriage at Frogmore House. For the seating chart at their reception, the bride and groom followed the American tradition of placing married couples beside one another. However, Harry’s brother and sister-in-law, Prince William and Kate Middleton, respectively, were supposedly not very keen on this arrangement—and they even took matters into their own hands to make some adjustments that better suited their preferences, according to Harry’s memoir, Spare.

Based on the Duke of Sussex’s book, it appears that the couples’ expectations over the affair—and the traditions it maintained—clashed. At the reception, Harry and Meghan had the Prince and Princess of Wales seated next to one another, which is typically the standard etiquette at American nuptials, but the two allegedly wanted to maintain the British wedding tradition of sitting apart from one another.

To resolve this issue, William and Kate supposedly switched up their seats—an act that irked Harry. “For our part, we chipped in that we weren’t too pleased when Willy and Kate switched place cards and changed seats at our wedding,” Harry writes. “We’d followed the American tradition, placing couples next to each other, but Willy and Kate didn’t like that tradition, so their table was the only one where spouses were apart.”

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According to the forthcoming memoir, when Harry confronted William and Kate about this incident, they claimed another person must have changed their place cards. Then, the duo accused Harry and Meghan of altering the seating chart at Pippa Middleton and James Matthew’s wedding on May 20, 2017. “We hadn’t,” Harry insists in his book. “Much as we’d wanted to. We’d been separated by a huge flower arrangement between us, and though we’d desperately wanted to sit together, we hadn’t done a thing about it. None of this airing of grievances was doing us any good, I felt. We weren’t getting anywhere.”

Etiquette expert William Hanson responded to this bold move in his book, Just Good Manners, writing that even though the tradition might have felt out of place at a primarily British event, the act is still unacceptable for any guest. “For what it’s worth, while you could question the logic of a supposed American tradition being followed when the event was in Britain and a large majority of the guests were British, it is very poor form to switch place cards without permission from the hosts, whoever you are,” he declares in his written work.

This wasn’t the first time the two couples argued during the wedding-planning process. In another squabble, Meghan reportedly accused her then-pregnant sister-in-law of having “baby brain,” which caused William to jump to his wife’s defense. According to biographer Robert Jobson, William pointed his finger at Meghan before saying, “Well, it’s rude, Meghan. These things are not done here,” to which Meghan replied, “Take your finger out of my face.”