zsh-completion file for mercurial is installed as /usr/share/doc/mercurial-${PV}/contrib/zsh_completion, regardless of USE-flag "zsh-completion". And when mercurial is installed with zsh-completion: % qlist mercurial | grep zsh /usr/share/doc/mercurial-9999/contrib/zsh_completion /usr/share/zsh/site-functions/_hg Files are identical, of course. Reproducible: Always
...the same thing goes for Emacs files and lots of other contrib stuff, like RPM .spec etc.
Fixed in 2.5.4 and 9999, thanks Nikolaj