Ask HN: What tricky questions you were asked during Ruby/Rails interview?

For folks who were interviewed for Ruby (and Rails) developer positions: what technical questions you were asked? Especially the ones that involved writing some code. Like algorithm/data structure puzzles or something like this (taken from rubyinside) "Take the result from http://api.twitter.com/version/statuses/public_timeline.json and output an array that meets the following criteria: has usernames as elements that has the text 'Ruby' somewhere in the tweet ordered by follower count of user who posted"