The current cache busting strategy for static assets on the portal uses a file-hash suffixed to the end of a filename, such as index-309ffl4.js. If this file is improperly cached, it could lead to a 404 page instead.
Using a cache-busting solution such as index.js?309ffl4would prevent this issue.
It should be noted that this could lead to a situation where the JS file is expired but the HTML is not, leading the HTML to load a newer version of the JS file than was intended. This however, is still preferable than having the JS file return a 404.