I\'ve had problems with autotest before which I resolved by downgrading from ZenTest 4.1.4 to 4.1.3.
$ ruby -v ruby 1.8.7 (2008-08-11 patchlevel 72) [univers
I had the same problem with Shoulda tests, downgrade to 4.0.0 solved it.
sudo gem uninstall ZenTest
answer yes to the remove binaries question
sudo gem install ZenTest --version '4.0.0'