http://www.google.com/intl/en/webmasters/+1/button/
the above discussion would be useful, but not at all.
by myself experience, if the current page which is placing google plus button wouldn’t be visited by google crawler, or the web page has some errors special for google crawler, this will cause displaying red flag with this button. typically, that web page can be fetched by a real web browser, but not for google web crawler. in general, web crawler won’t attach more http request headers! so if your web page need to check specific http request headers, and raising exception by this checking, that should be the reason!
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Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect traffic, set breakpoints, and “fiddle” with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, and can be extended using any .NET language.
Fiddler is freeware and can debug traffic from virtually any application that supports a proxy, including Internet Explorer, Google Chrome, Apple Safari, Mozilla Firefox, Opera, and thousands more. You can also debug traffic from popular devices like Windows Phone, iPod/iPad, and others.
http://www.fiddler2.com/fiddler2/
After you start Fiddler, the program registers itself as the system proxy for Microsoft Windows Internet Services (WinInet), the HTTP layer used by Internet Explorer, Microsoft Office, and many other products. You can verify that Fiddler is correctly intercepting requests by checking the Proxy Settings dialog. From the Internet Explorer main menu, click Tools, click Internet Options, click Connections, click LAN Setting, and finally click Advanced.
更多细节看：
http://msdn.microsoft.com/en-us/library/Bb250446.aspx
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