At work, we've got to use a proxy to connect to the internet. Sometimes, I get the following error message from the proxy: Your client has attempted to perform a request in a manner not allowed by the HTTP protocol standard. If you are using Microsoft Internet Explorer, make sure the "Use HTTP 1.1 through proxy connections" option is disabled, and click reload to try again. This error may also be caused by using MIE version 4 without Service Pack 2 (or HotFix #2798) installed. When I set the HTTP version to 1.0 at Debug->Networking, I do not get this error message. However, since this setting is at the Debug prefs pane, I "fear" that the setting will go away once a final will be released. Because I suppose quite some people might need this setting, it should always be accessible thru the prefs dialog.
Bug 38488 discusses this issue and the possibility of a new pref in the proxy dialog for HTTP 1.0 compatibility. *** This bug has been marked as a duplicate of 38488 ***