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.
I am personally using mochaUI is my current projects, and it was one of the main reasons a started working with MooTools at all, but this alternative to a web based desktop implementation is also something to pay attention too.
Although MochaUI is far more feature-packed, i think that this basic JQuery implementaion can be a good start for starting off a new web based desktop application (Add some plugins, and ding! ready).
You're seeing this error because I think it is funny.