Taking to Instagram, the Underworld star shared a series of photos from her sick bed with her 2.5 million followers.
The post prompted an outpouring of support from fans, quickly racking up more than 330,000 likes and garnering coverage from international media.
But when Beckinsale stumbled across an article from USA Today, she spotted a serious slip up in the cover photo.
Reporting on her illness, the outlet had used a picture of a different Kate - namely Catherine, Duchess of Cambridge.
Seeing the funny side of the gaffe, the 45-year-old star posted a screenshot of the story to Instagram.
Using her trademark wicked sense of humour, she captioned the grab:
"So moved and touched by all the kind wishes I have received over the last few days.
"Back to resume my royal duties. William says hi."
We can only dream of the day we are mistaken for HRH!