I use Adobe BrowserLab to test my sites in multiple browsers, recently I've discovered all my SilverStripe sites had stopped working in BrowserLab, I would only get this message:

This happens on sites that used to work fine in BrowserLab that haven't been touched, and even the SilverStripe demo site at demo.silverstripe.org.

So I posted on the BrowserLabs forum, and here is a reply from their developer:

"When I preview http://demo.silverstripe.org/ on my dev browser server errors are generated when decompressing the main page - java.io.UnsupportedEncodingException: "UTF-8". I checked another page that uses gzip compression and UTF-8 encoding and it works fine - http://shop.cascade.org/. This leads me to believe that there perhaps is a problem with the encoding on the pages this person is previewing."

Their full reply is here: http://forums.adobe.com/message/3696408#3696408

So is this a SilverStripe bug?

I don't think it can be the actual contents of the page, because BrowserLab works fine with static content generated for the same page. (get the dynamic version for the same page by adding ?1=1 to the url)