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 you use some of your ajax helpers, such as link_to_remote, with the :update parameter in order to update a specific DOM element AND you specify page.replace_html or any other RJS syntax that handles other DOM ID, it will return the result expected, nothing will be updated on the :update pointer DOM ID.
Simply choose one, if you use :update, return an html or render a partial in your action instead of rendering RJS.
If you require multiple changes, use RJS and no :update parameter.
You're seeing this error because I think it is funny.