SVN 1.5 changed the Subversion help output to print ARG instead of arg. Unfortunately, the file which ZSH uses to parse the Subversion help output to generate the tab completion options was only looking for a lower case arg. The result is the error _arguments:comparguments:208: invalid argument: ARG when trying to tab-complete svn commands in ZSH.

The fix was committed to the ZSH source in Jan 2008, but still has yet to make its way into all the machines being sold by hosting providers.

If you get stuck with a box with the new SVN but the old ZSH you can fix this yourself by editing /usr/share/zsh/x.x.x/functions/_subversion and replacing

arg/:arg:

with

(arg|ARG)/:arg:

both places it occurs.

Voilà! SVN tab completion works again.