Not sure if you are running zsh version 5.0.0 but I've recently run across an issue with opening a new shell with the error above. It appears to be coming back down to the __git_complete. I am not for sure what has caused this problem to start but here is the dump from script zsh -x https://gist.github.com/4212585.
script zsh -x
(Search that link for "complete:13" and it will take you right to the first error)
This is related to (and should be fixed by) #43.