Shopify says 'continue selling when out of stock' turned on by itself — stock went negative
Nobody flipped this switch in the admin — it came from a column in your import file. Here's the setting behind it, and the spreadsheet habit that carries the wrong value into rows that were never meant to have it.
What you'll see
A product's inventory drops below zero — customers keep checking out even though the count says there's nothing left. In the admin, the variant's inventory policy shows "Continue selling when out of stock," but no one on your team turned that on.
The column behind the setting
This behavior is controlled by the Variant Inventory Policy column in Shopify's product/inventory CSV, which accepts two values: deny stops sales the moment stock hits zero; continue lets checkout proceed and the count goes negative. Whatever value is in that column when you import overwrites the current setting for that variant, the same way other inventory columns do.
How the wrong value ends up in your file
The most common source is spreadsheet mechanics, not intent. Duplicating an existing row to create a new product row carries over every column's value, including the policy — if the original row was set to "continue" (common for backorder or pre-order items) and you forget to change it back, the new product inherits overselling. Excel's fill handle causes the same problem: dragging a cell down to fill a column of "deny" values will just as easily drag a "continue" value down into product rows that should never have it, since the fill handle doesn't know which rows are which product.
How to check and fix it
Scan the Variant Inventory Policy column for any row that shows continue and confirm that's actually intentional for that specific SKU (pre-order items, made-to-order goods). Anywhere it isn't, correct it to deny before re-uploading, and avoid fill-dragging that column across rows belonging to different products.