Ruby

Quoting the first line of http://ruby-doc.org/stdlib-2.3.1/libdoc/set/rdoc/Set.html:

Set implements a collection of unordered values

This would mean the set {1,2,3} is exactly the same as the sets {1,3,2}, {2,1,3}, {2,3,1}, {3,1,2} and {3,2,1}. This is a property of the mathematical concept of sets (see https://en.wikipedia.org/wiki/Set_%28mathematics%29, which also describes the order of the items as being irrelevant). So actually, I don't see how using Set#find_index would make any sense. The current implementation just accidentally preserves the order of the items in the set via undocumented behaviour, relying on that would be scary.