When trying to update my #Debian box today, it would not work because some third-party PGP key had expired:

Err:5 https://deb.librewolf.net bookworm InRelease
  The following signatures were invalid: EXPKEYSIG D29FBD5F93C0CFC3 Malte Jürgens <maltejur@dismail.de>
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://deb.librewolf.net bookworm InRelease: The following signatures were invalid: EXPKEYSIG D29FBD5F93C0CFC3 Malte Jürgens <maltejur@dismail.de>
E: Failed to fetch https://deb.librewolf.net/dists/bookworm/InRelease  The following signatures were invalid: EXPKEYSIG D29FBD5F93C0CFC3 Malte Jürgens <maltejur@dismail.de>
E: Some index files failed to download. They have been ignored, or old ones used instead.

Turns out this is caused by my favourite web browser #LibreWolf which I installed many months ago. In the meantime, the recommended installation method for Debian has changed. So what I had to do to fix this was to remove one file …

sudo rm /etc/apt/sources.list.d/librewolf.sources

… and after that, follow the new installation instructions for Debian.

PS: You should remove more files, LibreWolf's Debian Migration Guide says. I only discovered it now.