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:
- Hit "save" if you still have unsaved changes to your product
- Hit "Categories" and click on the involved category
- Scroll down to the bottom to see all the products assigned to this category
- Delete all the unfitting products via click on the cross
- Save and you are good to go :-)