You are in the editing mode of your product under "Category assignment" and chose a category for your product by mistake? This is how you undo it:
1. Hit "Save" if you still have unsaved changes to your product.
2. Hit "Categories".
3. Click on the involved Category.
4. Scroll down to the bottom to see all the products assigned to this category.
5. Delete all the unfitting products via clicking on the ''remove'' icon.
Save and you are good to go :-)