The cart cookie can cause issues if a product type is changed after it has been added to the cart.

• User adds the product to their cart which is then stored in a cookie.  Product is of type Multi
   
• Administrator changes product type from Multi to Attribute
• User comes back to site which restores their cart from their cookie
• Product is still stored in the cart as a Multi product and not updated to type attribute

No immediate fix is available.  You can turn of the cookie feature or ensure that your products are correctly configured before publishing them.