Editing a supervised dedupe rule to make it unsupervised and vice versa eliminates the original dedupe rule

This is a subtle problem that causes users to create messes that they can't figure out.

If you edit an unsupervised rule (reserved or not) and save it as the supervised rule, you end up with no unsupervised rule.
If you edit a supervised rule (reserved or not) and save it as the unsupervised rule, you end up with no supervised rule.

If you don't do something about it and go about your merry business, you'll have crashing online event registrations. I duplicated this on drupal.demo.civicrm.org.

Since you are required to have a supervised rule and an unsupervised rule to run CiviCRM, I think it would be better if the UI prevented users from doing this.

On a related note, I think that the UI should not permit a threshold of 0. When users get lost in dedupe rules, this error often accompanies the other problems.