When a Mac OSX buildbot slave runs cvs, it behaves differently if a directory named "cvs" is found on the path.
For example,
$ export | grep PATH
declare -x PATH="/foo/cvs:/opt/local/bin:/tools/buildbot/bin:/tools/twisted/bin:/tools/twisted-core/bin:/tools/python/bin:/opt/local/bin:/opt/local/sbin:/bin:/sbin:/usr/bin:/usr/sbin"
...will cause buildbot to attempt to run a program called "/foo/cvs", which doesnt work. Instead, buildbot should run a program called "/foo/bar/cvs/cvs".
To work around this, rename "/foo/cvs" to be "/foo/bar", adjust the PATH to match, and restart buildbot slave. Now, buildbot will run a program called "/foo/bar/cvs", which works.