Only tested in development, but until the Feature model has been loaded, any call to a Flip.on? :blarg will raise No feature declared with key :blarg.
If you simply place require 'feature' before a call, then the problem will go away (even for subsequent requests AFTER removing the require 'feature' line).
Is it easy enough to include a load method in Flip, which checks for initialization and requires 'feature' if it's not available?