Do you run Mac OS X and ZSH? Does your shell spew out readlink errors for some commands? Like this?
$ umount /Vo*tab*readlink: illegal option -- q
usage: readlink [-n] [file ...]
readlink: illegal option -- q
usage: readlink [-n] [file ...]
readlink: illegal option -- q
usage: readlink [-n] [file ...]
...That’s because ZSH assumes you’ve got the GNU version of readlink. Install that version with MacPorts (port install coreutils), apply this patch to your system and the problem will belong to your past.