On Windows, all the usual browsers.

Tried Windows 7, Chrome/IE/Firefox - can't reproduce. If it affects IE and Chrome (which have almost no code in common), I'm guessing you are behind a proxy server of some sort, so I'm going to check my Caching / Vary headers. Thanks for the pointers!

Corporate firewall when it happened to me.

Thanks for confirming. I think you've found an issue with haproxy compression; compression support is quite new, and there are still issues being fixed. I'm going to move compression into the web server layer (it probably belongs there anyway!)