Saw this in arch-dev-public, pasting it:
The nss and lib32-nss packages prior to version 3.51.1-1 were missing a soname link each. This has been fixed in 3.51.1-1, so the upgrade will need to overwrite the untracked files created by ldconfig. If you get any of these errors nss: /usr/lib/p11-kit-trust.so exists in filesystem lib32-nss: /usr/lib32/p11-kit-trust.so exists in filesystem when updating, use pacman -Syu --overwrite /usr/lib\*/p11-kit-trust.so to perform the upgrade.
Got it 10 minutes ago on my Archlinux + testing main installation.