The Bond star was attending a function at the royal residence in London with fellow actor Sir Ian MCKellen when they sneaked into a side-room to change outfits.
In the darkened room, Dame Judi sat down to adjust her clothing before realising the chair she was in had a tall back and was shaped like a sovereign's seat.
She tells U.K. talk show host Graham Norton,"I was at a party with Ian MCKellen at Buckingham Palace. I can't remember why we were there but we slipped into a room to change and we were feeling around in the dark and he suddenly felt something and said, 'Good grief, this is a very high chair, with a great tall back'. I think we were sitting on a throne!"