>> require 'pathname'
>> Pathname.new('/').exist?
=> true
>> Pathname.new('/').children.first.exist?
=> false
>> Pathname.new('C:\\').children.first.exist?
=> true

This is maybe a little obscure but Java considers C:\ to be equivalent to *nix root. And pathname seems to as well, but anything under root isn't recognized. Having this problem in the File class as well, just not as pretty to demo.

Tested under 1.7.12 and 1.7.9 with Java 8 and Win7. Problem exists in all compatibility modes as well.