Even before they exchanged "I Dos" to mark the beginning of their marriage life, Prince Charles broke Princess Diana's heart by doing something ridiculous in front of her.

In November 1977, the then-16-year-old Princess Diana met the heir to the throne, who happened to be dating her older sister. Prince Charles saw Princess Diana as her potential bride, and their relationship progressed since then.

Despite seeing each other for just 12 times, Prince Charles proposed to Diana in February 1981 at Windsor Castle.

Because of the "too abrupt" wedding proposal, Princess Diana had it worst, most notably when she felt like her future husband's heart belonged to someone else already.

How Did Charles Broke Diana's Heart?

A few weeks later after proposing, as they were in in the middle of wedding planning,  Prince Charles had to continue his royal duties and had to travel to Australia and New Zealand. 

However, before the Prince of Wales jetted from the country, Princess Diana admitted years later that her then-soon-to-be-husband did something that broke her heart.

Royal expert Anthony Holden revealed in his book "Charles: A Biography" that when Princess Diana was about to bid her goodbye to Prince Charles, the phone rang. Apparently, the mysterious call came from Camilla Parker Bowles.

The royal prince, instead of ignoring the call and focusing on his bride first, picked up his phone and completely snubbed Princess Diana.

Holden said that the Princess of Wales walked out of the room crying since the conversation appeared "so tender" that she felt like she needed to leave. After she set foot outside the room, that was when Prince Charles talked to her.

"[Diana] was weeping because her last minutes alone with the prince at the palace had been interrupted by a phone call," the royal expert and author said. "That moment, she said 10 years later, 'just broke my heart.'"

Not a Fairy Tale

Aside from the heartbreaking moment which Princess Diana only revealed years later, royal watchers already saw some signs that everything between the royal couple was not like those stories in a fairy tale book.

Before the royal wedding, Prince Charles and Princess Diana appeared in an interview with Former BBC News Anchor Angela Rippon.

The Queen's press secretary Michael Shea warned them that the interview could be interrupted and ended if the anchor would ask any question which they never agreed on. Michael stood behind the news anchor to make sure that everything would be smooth sailing.

Shea reportedly told then 20-year-old Princess of Wales to "get it right" since they were sitting in front of the television camera and millions of people were watching them.

Prince Charles and the press secretary looked very protective of the princess since she was "almost slumped in the chair" during the interview.

"I'm not quite sure how comfortable it made her feel," Rippon said as she watched the interview again for Channel 5 documentary's "Charles and Di: The Truth Behind their Wedding."

"With the knowledge of hindsight, you read so much more into body language, what was said, looks that were given, reactions. That's the value of hindsight, isn't it," she went on. 

READ MORE: Royal Heartbreak: Did Queen Elizabeth II FORCE Princess Diana to Divorce Prince Charles?