Martian Intuition

Response code "406" makes it look like a web server error, but if you're troubled in Windows Internet Explorer 6, it may very well be a client-side error.

Client-side Causes

• loading a behavior in CSS, like img.transparent { behavior: url(iepngfix.htc); }, where the URL actually returns an error (self-diagnosed in Windows Internet Explorer 6)
• broken meta tags (source)

Server-side Causes

• Accept-headers that do not declare any types that the server can provide (source)
• over-zealous mod_security (source)