There must be something wrong with the litter box/litter or the ferret. If you haven't changed anything then i would say maybe the litter box needs cleaning? Otherwise i would say there might be something wrong with your ferret, acting weird? poopies soft?
Maybe your ferret just couldn't make it that one time, has it happened several times? Have you tried doing the litter potty training again? Give him praise or a treat whenever he uses the bathroom and when he doesn't and you catch him, then quickly try to place him in the litter box.
Did you make sure you cleaned how the areas that he missed in with a good cleaner, so there isn't any smell leftover?
That is all i can think of at the moment. I had this problem twice, once was between the litter box needed cleaning. The second time was because i moved around their cage and they had accidents while they were playing, soon as i added a box where they were pooping, problem vanished.