Django JavaScript loading fails irregularly
Try disabling Chrome extensions and see if that helps. I had the same problem and it turned out to be Adblock extension that caused the problem.
UPDATE: According to @jessykate the full solution appears to be also to update Chrome to the latest version — at least 23.0.1271.91.
A 304 error means that a file has not been modified, so your computer doesn't need to download that file from the server. So it's not an error at all - it's just an indication that your browser is being more efficient.
If you've changed the file recently and want to see the changes, try clearing your cache.