Redmine

To reproduce:

1. Create a task A
2. Creata another task B and set A as parent: B -> A
3. Create a third task C and set B as parent C -> B -> A

Suppose B and C were both supposed to be children of A:

1. Edit C, change the parent field to refer to A instead of B
2. Save fails.
3. Edit C, change the parent field to nothing.
4. Save succeeds
5. Edit C, change the parent field to A
6. Save succeeds

So it is only possible to make C a child of A by first removing it from B. There seems to be no justification for this?