Prince Andrew's gaffe on Princess Beatrice's birthday

Princess Beatrice turned 31 on Thursday, and her family has taken to social media to wish her the happiest of birthdays.

Father Prince Andrew shared a collection of photos with his eldest daughter... but eagle-eyed fans noticed a rather embarrassing gaffe.

One of the photos Prince Andrew shared wasn't of Princess Beatrice at all, but rather his younger daughter, Princess Eugenie.

On his official Twitter account @TheDukeOf York, he added four photos with the caption, "Happy Birthday Princess Beatrice. Thank you to everyone for the lovely birthday wishes!"

The fourth photo featured Prince Andrew with his younger daughter, Eugenie, on her wedding day.

It was quickly deleted and replaced with a photo featuring Beatrice.

Sarah Ferguson, Duchess of York, chose a photo of Princess Beatrice as a newborn, a toddler playing in a sandpit, and as a grown up with the family's Norfolk terriers.

Younger sister Eugenie added six images of Beatrice, revealing her sweet nickname for her older sibling in the caption.

"You have been bossing it since before I was born and continue to be the most wonderful person, friend and big sister," she wrote.

"Happy Birthday to you Beabea!! Xx"

The official account for the Queen and Prince Philip also added a tribute to Beatrice for her birthday with the caption, "Today is Princess Beatrice of York's Birthday.

"Princess Beatrice is the first child of @hrhthedukeofyork and Sarah, Duchess of York, and fifth grandchild of The Queen and The Duke of Edinburgh.  #HappyBirthdayHRH."