I have found what I believe to be a bug in handling ordered subtasks.
Here is the behavior:

I have a top level set of tasks that is ordered.

One of the outline items below the top level set is a grab bag of tasks
that will be performed in parallel.  So this task is NOT ordered
(ORDERED: nil).

The problem is that the blocking behavior from ordered tasks seems to be
inherited from the top level task list into the second level of the
outline, even though the ORDERED property at the second level is
explicitly overridden.

I am attaching an org file that displays this issue.  To see the
problem, put your cursor on the "Bar" task and attempt to change its
status to DONE.

Cheers,
R