Make sure you are following the Same Origin Policy. This means same domain, same subdomain, same protocol (http vs https) and same port.
How does pushState protect against potential content forgeries?
EDIT: As @robertc aptly pointed out in his comment, some browsers actually implement slightly different security policies when the origin is file:///
. Not to mention you can encounter problems when testing locally with file:///
when the page expects it is running from a different origin (and so your pushState
assumes production origin scenarios, not localhost scenarios)