In demo you are expected to render a partial. When I include the rendering module, this works. When I don't I get the template engine exception.
As long as you either
a) include the rendering module as shown here for Padrino 0.9.26 (which simply makes the render method more powerful)
b) use an explicit engine as shown partial("foo", :engine => :erb) in edge padrino after this commit right now
partial("foo", :engine => :erb)
Note that b only works after changes I just made in Padrino so you must work off git because in edge padrino, I also added another way to get partials working without the need for augmenting rendering:
Option A works with stable gem Padrino. By adding rendering module you can also do things like this:
render 'foo' # rather than erb :foo
without an engine and Sinatra will autodetect that for you along with smarter rendering overall.