This happens if you use the Apache (or Nginx) Module “PageSpeed” from Google which creates combined and compressed and optimized versions of your ressources.
The problem is, they are not saved in your own folders, but are written into PageSpeed folders and used with ReWrite rules to access them.
SO far so good. But if you use Cache Everything or any other ReverseProxy Caching mechanism infron of your page which is also using PageSpeed this can cause trouble as your Cache is not flushing the cache when PageSpeed is clearing/changing its cache.
Thats why in @DotMrCode screenshot just ressources with the string
pagespeed in its name are getting 404 errors as PageSpeed already flushed the cache.
There are various options to work around this but none of these are a real solution thats why I would recommend to disable PageSpeed with this: