This morning I tried updating my ArchLinux server at home and stumbled into an issue where it wasn't able to find the PGP public key of one of the developers:

[root@shuttle ~]# pacman -Syu
:: Synchronizing package databases...
 core is up to date
 extra is up to date
 community is up to date
:: Starting full system upgrade...
resolving dependencies...
looking for conflicting packages...

Packages (33) archlinux-keyring-20200603-1  bison-3.6.3-1  ca-certificates-mozilla-3.53-1  conmon-1:2.0.17-1  cryptsetup-2.3.3-1  dbus-1.12.18-1  device-mapper-2.02.187-3  git-2.27.0-1
              glib-networking-2.64.3-2  gnupg-2.2.20-4  gnutls-3.6.14-1  go-2:1.14.4-1  iana-etc-20200601-1  iproute2-5.7.0-1  iptables-1:1.8.5-2  libgpg-error-1.38-1  libksba-1.4.0-1  libnghttp2-1.41.0-1
              linux-5.6.15.arch1-1  linux-firmware-20200519.8ba6fa6-1  lvm2-2.02.187-3  openssh-8.3p1-1  pacman-mirrorlist-20200606-1  pciutils-3.7.0-1  perl-5.30.3-1  pkgconf-1.7.3-1
              python-setuptools-1:47.1.1-1  runc-1.0.0rc90-1  slirp4netns-1.1.1-1  sqlite-3.32.2-1  systemd-245.6-4  systemd-libs-245.6-4  systemd-sysvcompat-245.6-4

Total Installed Size:  1294.85 MiB
Net Upgrade Size:        28.49 MiB

:: Proceed with installation? [Y/n] Y
(33/33) checking keys in keyring                                                                                            [##########################################################################] 100%
downloading required keys...
:: Import PGP key 3B94A80E50A477C7, "Jan Alexander Steffens (heftig) <heftig@archlinux.org>"? [Y/n] Y
error: key "3B94A80E50A477C7" could not be looked up remotely
:: Import PGP key 3B94A80E50A477C7, "Jan Alexander Steffens (heftig) <heftig@archlinux.org>"? [Y/n] Y
error: key "3B94A80E50A477C7" could not be looked up remotely
error: required key missing from keyring
error: failed to commit transaction (unexpected error)
Errors occurred, no packages were upgraded.

The key 3B94A80E50A477C7 wasn't found anywhere.

The fix is to first update the archlinux-keyring package first which contained the key, and then proceed with the normal update.

[root@shuttle ~]# pacman -S archlinux-keyring
resolving dependencies...
looking for conflicting packages...

Packages (1) archlinux-keyring-20200603-1

Total Installed Size:  1.23 MiB
Net Upgrade Size:      0.01 MiB

:: Proceed with installation? [Y/n] Y
(1/1) checking keys in keyring                                                                                              [##########################################################################] 100%
(1/1) checking package integrity                                                                                            [##########################################################################] 100%
(1/1) loading package files                                                                                                 [##########################################################################] 100%
(1/1) checking for file conflicts                                                                                           [##########################################################################] 100%
(1/1) checking available disk space                                                                                         [##########################################################################] 100%
:: Processing package changes...
(1/1) upgrading archlinux-keyring                                                                                           [##########################################################################] 100%
==> Appending keys from archlinux.gpg...
==> Locally signing trusted keys in keyring...
  -> Locally signing key D8AFDDA07A5B6EDFA7D8CCDAD6D055F927843F1C...
  -> Locally signing key DDB867B92AA789C165EEFA799B729B06A680C281...
  -> Locally signing key 91FFE0700E80619CEB73235CA88E23E377514E00...
  -> Locally signing key 0E8B644079F599DFC1DDC3973348882F6AC6A4C2...
  -> Locally signing key AB19265E5D7D20687D303246BA1DFB64FFF979E7...
==> Importing owner trust values...
==> Disabling revoked keys in keyring...
  -> Disabling key 8F76BEEA0289F9E1D3E229C05F946DED983D4366...
  -> Disabling key 63F395DE2D6398BBE458F281F2DBB4931985A992...
  -> Disabling key 50F33E2E5B0C3D900424ABE89BDCF497A4BBCC7F...
  -> Disabling key 27FFC4769E19F096D41D9265A04F9397CDFD6BB0...
  -> Disabling key 39F880E50E49A4D11341E8F939E4F17F295AFBF4...
  -> Disabling key 8840BD07FC24CB7CE394A07CCF7037A4F27FB7DA...
  -> Disabling key 5559BC1A32B8F76B3FCCD9555FA5E5544F010D48...
  -> Disabling key 0B20CA1931F5DA3A70D0F8D2EA6836E1AB441196...
  -> Disabling key 07DFD3A0BC213FA12EDC217559B3122E2FA915EC...
  -> Disabling key 4FCF887689C41B09506BE8D5F3E1D5C5D30DB0AD...
  -> Disabling key 5A2257D19FF7E1E0E415968CE62F853100F0D0F0...
  -> Disabling key D921CABED130A5690EF1896E81AF739EC0711BF1...
  -> Disabling key 7FA647CD89891DEDC060287BB9113D1ED21E1A55...
  -> Disabling key BC1FBE4D2826A0B51E47ED62E2539214C6C11350...
  -> Disabling key 4A8B17E20B88ACA61860009B5CED81B7C2E5C0D2...
  -> Disabling key 5696C003B0854206450C8E5BE613C09CB4440678...
  -> Disabling key 684148BB25B49E986A4944C55184252D824B18E8...
  -> Disabling key 8CF934E339CAD8ABF342E822E711306E3C4F88BC...
  -> Disabling key F5A361A3A13554B85E57DDDAAF7EF7873CFD4BB6...
  -> Disabling key 5E7585ADFF106BFFBBA319DC654B877A0864983E...
  -> Disabling key 65EEFE022108E2B708CBFCF7F9E712E59AF5F22A...
  -> Disabling key 40440DC037C05620984379A6761FAD69BA06C6A9...
  -> Disabling key 34C5D94FE7E7913E86DC427E7FB1A3800C84C0A5...
  -> Disabling key 81D7F8241DB38BC759C80FCE3A726C6170E80477...
  -> Disabling key E7210A59715F6940CF9A4E36A001876699AD6E84...
  -> Disabling key 5357F3B111688D88C1D88119FCF2CB179205AC90...
  -> Disabling key 4D913AECD81726D9A6C74F0ADA6426DD215B37AD...
  -> Disabling key FB871F0131FEA4FB5A9192B4C8880A6406361833...
  -> Disabling key 66BD74A036D522F51DD70A3C7F2A16726521E06D...
  -> Disabling key B1F2C889CB2CCB2ADA36D963097D629E437520BD...
  -> Disabling key 9515D8A8EAB88E49BB65EDBCE6B456CAF15447D5...
  -> Disabling key 76B4192E902C0A52642C63C273B8ED52F1D357C1...
  -> Disabling key 40776A5221EF5AD468A4906D42A1DB15EC133BAD...
  -> Disabling key D4DE5ABDE2A7287644EAC7E36D1A9E70E19DAA50...
  -> Disabling key 44D4A033AC140143927397D47EFD567D4C7EA887...
==> Updating trust database...
gpg: next trustdb check due at 2020-10-17
:: Running post-transaction hooks...
(1/1) Arming ConditionNeedsUpdate...