She famously remarked that she had a "crowded" marriage to Prince Charles thanks to his affair with his now-wife, Camilla, Duchess of Cornwall.
And during their ten-year marriage, in which they both confessed to infidelities, Princess Diana didn't take his relationship with Camilla lightly.
In fact, as Diana's biographer Andrew Morton explains in his book Diana: Her True Story, the ladies once had a very terse encounter about The Prince Of Wales.
Listen to Diana talk about the heated confrontation in the video above
Back in 1989, Diana had had enough of Charles sneaking behind her back so she decided to show up - unannounced - to Camilla's sister's 40th birthday party.
Proving just how fearless she really was, Diana walked up to Camilla, Charles and another friend and asked to speak privately with Camilla.
"She looked really uncomfortable and put her head down and I said to the men: 'Boys, I'm just going to have a quick word with Camilla and I'll be up in a minute,' and they shot upstairs like chickens with no heads, and I could feel upstairs all hell breaking loose," Diana reflects in the book.
Despite feeling "terrified", Di calmly proceeded to confront the other woman in her marriage: "Camilla, I would just like you to know that I know exactly what is going on between you and Charles, I wasn't born yesterday."
"Deathly calm and I said to Camilla: 'I'm sorry I'm in the way, I obviously am in the way and it must be hell for both of you but I do know what is going on,'" the mother-of-two remembers.
In taped recordings about the exchange, Diana claims Camilla, who wed Charles in 2005, replied to her: "You've got everything you ever wanted. You've got all the men in the world (to) fall in love with you and you've got two beautiful children, what more do you want?"
To which she replied: "I want my husband."
Looking back on the events, Diana has said it was "one of the bravest moments" of her marriage.
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Following her ballsy confrontation, Diana said Charles was furious.
"In the car on the way back my husband was over me like a bad rash and I cried like I have never cried before it was anger, it was seven years' pent-up anger coming out. I cried and cried and cried and I didn't sleep that night."
Ultimately the royal couple called time on their tumultuous marriage and were divorced in 1996. Just a year later, Diana was tragically killed in a car crash in Paris.