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.
When working in a team, you must be able to understand and to complete/continue other people code, although rails itself is very easy to understand, some of us need some further instructions :).
You're seeing this error because I think it is funny.