Today, I've had two completely separate applications bite me in the arse regarding directory names not being pluralised when, at first glance, they should be. The only commonality between them is that they're both written in Python.
First, Xen, or rather the userspace management tools. The directory which stores the data for all running guest domains is /var/lib/xen/xend-db/domain.
Secondly, Zope 2.8. Where do all of the Zope instances you want to run live? /usr/lib/zope2.8/instance (or somewhere similar). Crackful.
Naturally, these are very minor points which get fixed with a quick "WTF? ls Gah!", but it's a curious thing -- like whether you name database tables for the singular or plural of the "thing" they store. (The correct answer is "who cares, as long as you're consistent"). The general consensus (from a quick, very non-scientific analysis) that there seem to be a lot of directories which contain a collection of individual, related items named in the plural form. For example:
/etc/alternatives /etc/modutils /etc/X11/Xresources /etc/X11/app-defaults /etc/X11/cursors /etc/X11/xkb/keycodes /etc/X11/xkb/semantics /etc/X11/xkb/rules /etc/X11/xkb/symbols /etc/X11/xkb/types /etc/X11/rstart/commands /etc/X11/rstart/contexts /etc/X11/fonts /etc/X11/ja_JP.eucJP/app-defaults /etc/X11/ko_KR.eucKR/app-defaults /etc/console-tools /etc/acpi/events /etc/sgml/gnome-doc-tools /etc/mkinitrd/scripts /etc/ppp/peers /etc/chatscripts /etc/udev/scripts /etc/joe/charmaps /etc/emacs /etc/defoma/hints /etc/fonts /etc/gconf/schemas
(I kept /etc/emacs in there because I figure there's no way anything that big can be a single item. <grin>)
So, let the umpteen billion counterexamples flood in. Remember, though, that I've got all of the /usr/share/doc/*/examples directories on my side...