No, this is not a real Sinatra error :).
This is the personal homepage of Elad Meidar, a web developer and an entrepreneur specializing in Ruby on Rails. I hang around in Israel, and i am currently having the best time of my life over at Fiverr.com
I am a proud member of RailsBridge, Helping new Rails developers get into our world and also contributed a few Patches to the Ruby on Rails core.
" There’s this slick HTTP header called “X-Sendfile” that Apache and Lighttpd support. It was made to address this kind of problem (isn’t that convenient?). Now, when you send an HTTP response, all you have to do is tack on an X-Sendfile header with the path of the file you need to sendódon’t worry about actually reading the file or sending any of those bytes yourself. The web server will load the file you specified and send it downstream."
You're seeing this error because I think it is funny.