Miley Cyrus and Liam Hemsworth’s December wedding was supposed to be a top-secret event, but one guest’s innocent slip up made sure it became one of the biggest entertainment stories in the world.

Miley’s older sister Brandi Cyrus has opened up about how her famous sibling’s plan for low-key nuptials was spoiled when one of Liam’s friends shared photos of the event on Instagram.

Speaking on her podcast "Your Favorite Thing with Wells & Brandi," Brandi explained how Miley had enforced a strict no photos policy at the December 23 wedding but unfortunately not everyone got the message.

The couple had invited only a “very, very small, tight-knit group of people” which included “a handful” of Liam’s Australian friends and Miley’s best friend Jessie.

An error occurred while retrieving the Instagram post. It might have been deleted.

“Even with family and close friends, they still made a point to say, ‘Hey guys, this is a private thing for a reason. We love you all, but please don’t take photos,’” Brandi said.

But unfortunately, Liam’s surfer friend Conrad Carr didn’t realize there was a no photos rule and shared a snap of the newlyweds cutting their wedding cake, tipping everyone off.

“I didn’t really ask what happened, but I think from what he said, no one told him not to post about it, so he didn’t know,” Brandi said.

While Conrad “feels really bad” about revealing the couple’s wedding, Brandi explained Miley “doesn’t really care” that the news got out.

The "Malibu" singer shared photos of the wedding to Instagram a few days after the happy occasion, along with a video of her and Liam dancing.

An error occurred while retrieving the Instagram post. It might have been deleted.

Miley and Liam have dated since meeting while filming Nicholas Sparks romance film "The Last Song" in 2009.

The couple split in 2013 but reunited in early 2016 during a holiday in Byron Bay with Liam’s older brother Chris Hemsworth.

The pair tied the knot in a small ceremony at their home in Tennessee surrounded by family and friends.

An insider close to the singer told People the marriage was brought on by the loss of their Malibu home during the California wildfires last year.

“They don’t have doubts about their relationship,” the source said. “They both wanted to get married. They have both matured a lot, and it seemed like the perfect time for them to get married.”

This article originally appeared in news.com.au.