Now this is an obscure bug: Autotest on Leopard will break when used with projects containing Rspec specs, because it can’t find the “spec” command in the right place. It doesn’t look in /usr/bin/spec, where Leopard places it.

I tried the tricky answer before I gave up and adopted the simple answer:

$ sudo ln -s /usr/bin/spec /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/spec