You have to keep updating it fairly often, otherwise things slip through the cracks. Most recently on a machine that hadn't been updated in about a year it wasn't able to install anything because it couldn't update its GPG keyring anymore. I find the solution to be pacman-keys --refresh-keys or something like that. Why they can't do that as part of one of the updates, I don't know.

There's also small things that crop up during normal installs but that's to be expected on any distro due to bugs in various packages.