To reproduce this bug do the following:

Create a template with low level discovery.
Link the template to a host.
Wait for it to create items.
Unlink the template using unlink (not unlink and clear)
Now the items remain, but they can't be deleted or edited because zabbix thinks they are attached to a template, but the template is no longer linked.

What it should do is allow the user to edit the items, or remove them, but having items that can't be edited through the template or the host is a significant usability problem.