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.
Specifying exception messages, even as regexps, unduly limits implementations from providing better exception messages or messages in other languages while at the same time not adding anything of significant value to code correctness. The only exception (which probably should not be an exception) is in the specs for #raise.