If this is happening a lot without the card being removed from the camera, the card or the camera are faulty.
However, I suspect this may be because the card was removed from the camera and put in (for instance) someone's macbook, which of course insisted on writing lots of metadata to it - thumbnails, previews, trash and other management data, and so on. Presumably this was done in such a way that the camera didn't like the resulting filesystem when the card was put back in it.
It was probably still possible to read the card in a computer at that point, but the camera might not have agreed to record to it until it had been reformatted to its liking.
I think the moral of the story is fairly clear. Your original camera media is not your video assist.