When creating a subtask, a check if the parent task exists, and a warning if not, would be nice.

e.g. https://secure.phabricator.com/maniphest/task/create/?parent=920000

I spotted this during a migration operation, where after a database reset, I clicked on the link of a cached task, not anymore present.

But that could also occur in regular create workflow, if someone tweaks the URL to quickly add a parameter not directly accessible in the create new task form.