So because Magento V1 is horrible at handling Configurable Products, a lot of us are forced to use the prodigal SCP Extension (Simple Configurable Products) which, much like the Ryugyong Hotel, just stopped being supported. Thankfully, a lot of the features still work, but there are some issues which have been fixed by the community.
One such issue is one I’ve encountered when I tried adding a quantity of product which exceeded the availability. Because the simple product isn’t visible in the store (it’s hidden because it’s associated with the configurable product), you’re taken to a 404 page. As far as Magento is concerned, the product doesn’t exist.
After weeks of fudging this (using plugins to cover it up basically), someone posted a fix on the plugins github:
Basically, all you need to do is append some code onto a couple of the plugin files and the plugin now redirects to the configurable product and NOT the out of stock hidden simple product.