Luis who has been adding his company's methodology to the platform, asked me to delete some metrics for him - because he created them to test the system, and he's not able to delete them. I tried deleting a few of them and immediately got rats in every case.
I found the error in the log. Short version: I think this is fixed in staging.
Notes for me: error was in metric type change triggering lookup update.
btw, this makes me very happy that all that refactoring I just did of the lookup handling may already be paying dividends :).
This appears to be working now.