Since 1.8 does not canonicalize the filename that is required, the same file can be required multiple time. This is particularly true for spec_helper.rb, however it could also affect execution of the specs. On 1.9, the path is canonicalized. Using File.expand_path should eliminate this difference on 1.8 and 1.9.
The auto conversion was a bug and fixed in trunk. [ruby-dev:40274]