问题
What is a cross-domain error?
回答1:
As I understand it, client-side tools such as Silverlight (and maybe Flash/Javascript) throw a cross-domain error when you attempt to make a connection to a server that is normally only allowed when it is made to the same domain that the page was served from (some origin policy).
A cross-domain error may be thrown when, for example, you are viewing a page on your test server when it is trying to call your live server, or when you are viewing a test page as a local file using a file:// protocol.
Try ensuring that the domain you are testing on is the same as that which the site was designed to be on. Note that Flash has the crossdomain.xml feature which specifically allow you to do cross-domain requests. Javascript also has ways to get around same origin policy, but you should be aware of the implications of what you're doing.
回答2:
It happens when Javascript (most of the time) try to access something which it shouldn't.
Such as if you try to read another domain's cookie, that won't work. If you try to do XMLHTTP request to another domain or protocol (HTTP > HTTPS) that won't work. Because if you can do that you can hijack, steal your visitors session in other websites.
It's security feature and now it's a standard in all browser.
来源：https://stackoverflow.com/questions/708519/cross-domain-error