jQuery recently started using source maps.
For example, let's look at the minified jQuery 2.0.3 file's first few lines.
/*! jQuery v2.0.3 | (c) 2005, 2013 jQuery Foundation, Inc. | jquery.org/license
//@ sourceMappingURL=jquery.min.map
*/
Excerpt from Introduction to JavaScript Source Maps:
Have you ever found yourself wishing you could keep your client-side
code readable and more importantly debuggable even after you've
combined and minified it, without impacting performance? Well now you
can through the magic of source maps.
Basically it's a way to map a combined/minified file back to an
unbuilt state. When you build for production, along with minifying and
combining your JavaScript files, you generate a source map which holds
information about your original files. When you query a certain line
and column number in your generated JavaScript you can do a lookup in
the source map which returns the original location. Developer tools
(currently WebKit nightly builds, Google Chrome, or Firefox 23+) can
parse the source map automatically and make it appear as though you're
running unminified and uncombined files.
emphasis mine
It's incredibly useful, and will only download if the user opens dev tools.
Solution
Remove the source mapping line, or do nothing. It isn't really a problem.
Side note: your server should return 404, not 500. It could point to a security problem if this happens in production.
