As you say yourself, 1.8.6 is slightly broken. But do you expect old releases to always be bug-free? 1.8.6 isn't even the newest version of the 1.8 branch, let alone that 1.9 is the current branch of Ruby.
And as for your distrust of the runtime, did you know that 1.9 is an entirely new one?
As I noted, 1.8.7 produces a hash collision for [2, 3] and [4, 5]. 1.9.1-p376 has exactly the same behavior. I reiterate: the Hash object is unreliable, regardless of the runtime version. Based on such nasty behavior in such a simple case, I distrust Ruby's runtime and standard library for more complicated uses.
And as for your distrust of the runtime, did you know that 1.9 is an entirely new one?