Prevent people from sneakily adding fields to your extension's reserved custom groups by using the Move action
- Make an extension that makes a reserved custom field group.
- Someone adds a field to some other non-reserved custom field group.
- Then they sneakily use the Move link from the links on the right to move it into your reserved group.
- You make some code update that then crashes or mangles the data in that field because it's not expecting it to be there.
The UI currently goes out of its way to prevent you even seeing that reserved groups exist, and bouncing if you try to hack an edit link to a field. It should also prevent moving a field into the group.