If you’re seeing a lot of “Error 412: precondition failed” errors in a site hosted at Joyent like this one it’s most likely the result of something triggering a rule in mod_security.
Add
SecFilterEngine Off
to .htaccess
btw: GigaOM in Coming Soon, Free Hosting for Facebook Apps contains “If you are a Facebook app developer, I have some good news for you: Joyent, a Marin County, Calif.-based on-demand computing and web hosting startup, is going to start offering free hosting to Facebook app developers.”