rake test fails under InstantRails but not OSX

I've found an intriguing issue with Rake. I develop under OSX and all
my tests pass when I run 'rake test'. I recently tried this on
Windows XP using InstantRails and a few of the unit tests failed.
Interestingly, if I manually execute rake_test_loader.rb with the same
parameters as Rake itself uses (or at least, what Rake prints to the
console), the tests pass fine, e.g:

$ ruby -Ilib;test "C:/InstantRails/ruby/lib/ruby/gems/1.8/gems/
rake-0.7.3/lib/rake/rake_test_loader.rb" "test/unit/test_1.rb" "test/
unit/test_2etc.rb"

Would anyone be able to enlighten me as to what Rake is doing
differently which might explain this behaviour?

Shot in the dark but if the locales for the database are different
from the windows box and the OS X box you could get failing tests if
you are testing the order of a result.

ex.
assert_equal Post.find_recent.first, posts(:first)

Its happened to me.
- Richard