It also seemed that Mary was not joined at the party by her husband Frederik, who was reportedly on a private trip abroad that weekend.
But that's not the reason why Mary's attendance at this particular private birthday event has caused a stir in Denmark.
Instead, it's because previous reports suggested the Queen had been explicitly warned to steer clear of her celebrity pal.
The reason? Caroline's 'party lifestyle' at the time was said to be a cause for concern.
Mary, who by then was Crown Princess, was reportedly told to be mindful of her future role as Queen, and reminded that certain associations could one day be seen as inappropriate for someone in her position.
So, could it be that Mary has been able to reconnect with her old pal now that she has a less controversial public image?
Or is it that Queen Mary has now reached a stage where she no longer feels the need to keep up appearances as she once did – particularly when it comes to her personal friendships and private time?
Is there room for her to enjoy a big night out at a dress-up party for a dear friend – despite being Queen?
As she grows increasingly confident and comfortable in her role, it seems the way forward is for Mary to do whatever she feels is right for her.
Posted by ButIDigress79