You have a product enabled in 2 stores, set the categories and the default category per store since the two store use different categories.
The problem occurs when for example try to edit the description of the product in "All stores" mode so I don't have to make the change twice.
I tick the checkbox next to the description editor and change the description. I don't change anything else. I press save and not only the description of the product changes but the default category of the product in both shops. So it basically changes the default category of the product in the 2nd shop to a category which isn't even enabled in that shop. This breaks multiple things, for example the breadcrumb at the product page.
If you encountered above problem, please check this link Its an unreleased commit, which seems solved the problem that i had above.