What we learned watching someone use the site for real.
Where it breaks
The review took longer than the fix, and that is the right way round. Most of the time went on working out what the code was meant to do and who depended on it. Once that was written down, the change itself was four lines, a test and a note in the changelog that future maintainers will actually thank us for.
A throwaway site is the cheapest tool in the kit. We can build one in a minute, break it completely and delete it without a single awkward conversation. Everything risky in this post happened there first, including the mistakes we are not going to mention, and nothing touched production until it had survived a full run from a clean start.
wp cron event list --fields=hook,next_run_relative
wp transient delete --expired
wp cache flush