How to catch a broken Shopify checkout before your customers do Shopify stores break quietly after app updates and theme edits: wrong variants in the cart, checkout errors, old prices, broken search. A 10-minute manual check, what to watch in analytics, and a free automated check.
Shopify adding the wrong variant to the cart: how to find the cause When a Shopify store adds the default size or colour instead of the one the shopper picked: compare the selected option, the ?variant= in the URL and the cart line. Theme customisations, cart apps and slow scripts are the usual causes.