Ok, in this case it’s because you have your developer tools configured to retrieve map files. Check your developer tools settings page (Developer tools / Settings / Preferences) in the “Sources” section. You’ll see entries for “Enable JavaScript source maps” and “Enable CSS source maps”.
Note that you’ll only get this error when the developer tools are open. If those tools aren’t open, no request is made for those map files. (In other words they don’t affect typical usage.)