Ruby's method_missing, the equivalent of Smalltalk's doesNotUnderstand:, is a plague on Ruby programmers. It might mean something returned nil, it might mean the wrong type got passed or returned, it might just mean that you forgot to include a module — but you will run into them more times than you can count while you're developing a Ruby app. (Or at least I will. I don't know. Maybe everybody else has better luck with their libraries or is just more studious than I am.)
