Why are some customers receiving wrong size orders from my online store?

Technical Q&A

Topic summary

Issue: Some orders intermittently record the smallest default size (XS) instead of the customer’s selected size, causing returns. Store uses a heavily customized Prestige theme; problem is hard to reproduce and most orders are correct.

Findings so far:

  • No clear JavaScript (JS) conflicts detected by a reviewer; a specific product URL was shared for testing (access may require AU VPN).
  • Hypothesis: a race condition during page load. If customers select a size before the page fully initializes, the default XS may be submitted because the variant state isn’t updated (missing/unchanged variant parameter in URL).

Recommended debugging/tests (proposed):

  • Temporarily hide or disable the add-to-cart (ATC) button until product-form JS finishes loading; longer-term, prioritize loading of product-form JS.
  • Simulate slow networks via throttling; test with/without ad blockers; try a fresh theme with app code removed to isolate interference.

Latest update from the merchant:

  • ATC is already hidden until full load; they could not reproduce adding before load.
  • With JS disabled or with an ad blocker, behavior is fine, which complicates the initial hypothesis.

Status: Unresolved/ongoing. Another merchant reports the same issue (multiple sizes, recent returns) and asks if a solution was found.

Summarized with AI on February 5. AI used: gpt-5.
4

90% this is happening because they are making a selection before the page has actually fully loaded.

When you confirm the basic testing steps below to establish ground-truth a quick fix can simply be to add a transition to the page that hides the add to cart until the relevant code is actually loaded.

A real fix is to optimize the theme to prioritize product form related javascript to load asap. It’s kinda nuts this isn’t the norm /shrug.

So whats going on here is the customers browser as the page is loading they literally can select “M” (medium) and see it is selected(and technically it is) and yet when they hit add to cart the XS(extra-small) is what’s actually added to the cart because it is the first item in the options and the currently selected one either in the theme, or what the javascript is set too when it loads just a bit before the AtC is hit.

This is most apparent when the variant parameter is missing or does not change i.e. product-name?variant=7654321

Other tests to run include disabling javascript to see what the behavior is when partially loaded, or fully loaded.

By partially loaded I mean making a selection and hitting the add to cart as soon as possible.

Or use developer tools to throttle the network connection to simulate a slow speed connection.

https://stackoverflow.com/questions/10328568/simulate-limited-bandwidth-from-within-chrome

And then for robustness doing the above with and without an adblocker running as this can be a factor too in javascript either not loading or being slow to load.

There’s also doing the above with a fresh install of the theme with {{ content_for_header }} commented out to remove app interference as a suspect. Note: some apps autoinstall files directly to a theme when it’s created making it harder to get a clean test /sigh.

FYI: There is no quantity shown on your cart page for items. If you do that on purpose and have a/b tested it as increasing conversions that is fascinating.