1. Phorge at fec00256be1f8b9133076b8d7329f0a99c891daa
2. Create a custom PhabricatorProjectTagsField class in Herald, for example by applying the patch in https://gitlab.wikimedia.org/repos/phabricator/phabricator/-/merge_requests/62/diffs
3. Set up a Herald Rule in Maniphest using this new custom condition
4. Remove your custom class again so your Herald rule will have problems

Expected results:

Some way for a Phorge admin to notice that there is an issue to be fixed with a Herald rule, e.g. via an entry in the server log or in DarkConsole.

Actual results:

Issue can only be realized by having the idea to go through Herald transcripts from time to time:

Other comments:

• This is a corner case but I'd say an additional phlog() won't hurt anyone?
• This is a a little bit related to T15869: Herald rule referring to a disabled rule silently fails.