While testing on staging this error was seen
Retry: update or delete on table "milestone" violates foreign key constraint "bugtask__product__milestone__fk" on table "bugtask"
DETAIL: Key (product,id)=(190,14857) is still referenced from table "bugtask"
When comparing the list of bugtasks between the milestone +index and milestone +delete, the private bugs were not listed on later.
This has never been seen in production because private bugs were not involved, and will rarely be involved since milestone deletion happens with mistaken (experimental) milestones rather than the genuine (production) milestones that was used on the staging test.
The RegistryDeleteViewMixin._getBugtasks() could use the principal of the current interaction, but that will not prevent this error from occurring if the owner or release manager does not have permission to see the bug. The correct fix must retrieve all bugtasks regardless of privacy. Private bugs that the user can see should be shown in the list. Those that cannot be seen will be silently untargeted. This implies that the untarget operation needs special access to get the list of bugtasks and need to use removeSecurityProxy to untarget the milestones. The user does not need to know that this happened.
This is not a high priority until this really happens.
Oops OOPS-1494F1826 shows that that can be experienced.