Pacman and "exists in filesystem" errors

Every now and then I try to update and am rebuffed because some file already exists and would be overwritten.

To find out what package owns the file: pacman -Qo /path/to/file

To install anyway and overwrite the existing file(s): pacman -S --force $package

Each installed package has a folder under /var/lib/pacman/local/ with metadata about the package.