Why can customers add a bundle to cart without choosing items?

There are a few possible reasons:

1. The quick view or quick add button on a product card. When you move your mouse over a product, you can add the product with a click. This is usually provided by the theme. But the buttons can be hidden by customizing the liquid file in the theme
2. The upsell widget on the thank you page, which is powered by 3rd party post-purchase upsell apps
3. The cart slider which offers upsell features
4. The cross-sell widget added to the existing product pages

How to fix this issue?

• Hide the widget with custom CSS or liquid files in the theme
• Exclude the bundle products (with product tags of 'BYOB display' and 'BYOB hidden') in the 3rd party apps
• You can activate the new cart validation feature with the new cart & checkout validation API. The wrong bundle SKU is blocked from adding to the cart. An error message will be shown. Customers can only build bundles on the BYOB bundle pages

How to activate the cart validation feature?

1. Visit Shopify admin => Settings => Checkout => Checkout rules
2. Add a new rule like the screenshot
   

   

3. Done. When the wrong bundle variant is added to cart, there will be an error message -
   
   You must select what products you like in the custom bundles!