是什么触发“Internet Explorer已修改此页面以帮助防止跨站点脚本。”？

I'm trying to implement a workaround for missing CORS functionality in Internet Explorer. For GET requests I use JSONP, no problem here. For small POST/DELETE/PUT requests I also use JSONP by tunneling the requests through GET but this does not work for larger requests (Because the length of the GET URL is limited). So for large data I try to implement a form POST via an iframe. I can't read the response from this POST because of the same-origin policy so I fetch the response via a JSONP GET request after posting the data. Works great but sometimes I get a strange warning in IE 9:I'm trying to implement a workaround for missin