Defining #method_missing and #respond_to? at the same time

I was reading Eloquent Ruby yesterday morning (buy a copy if you haven’t already), and it got me thinking about one of my “favorite” Ruby gotchas: defining #method_missing without a corresponding #respond_to?. E.g.:

The resulting code violates the Principle of Least Surprise, and often interacts in unexpected ways with other libraries. I started wondering […]