www-client/pybugz-0.8.0 emerged with USE="+zsh-completion" installs zsh functions in /usr/share/zsh/site-functions/_pybugz. Since pybugz runs in shell as /usr/bin/bugz, zsh functions cannot be utilized by zsh because of name mismatch. The solution would be doins'ing with other name (see "patch" attached).
Created attachment 225331 [details, diff] Fixes zsh completion for pybugz
Sorry for dumb report, that's problem on my system. Of course, filename for completion function does not matter, only function name.