[p4] Under what conditions can it say "File(s) couldn't be locked."?

There's a process here that tries to automatically merge-down changes
from a parent branch to a child branch: integrate, auto-resolve,
submit.  Sometimes the submit results in

Perforce.P4.P4Exception: File(s) couldn't be locked.
Submit failed -- fix problems above then use 'p4 submit -c #####'.

Under what conditions will a submit operation result in "File(s)
couldn't be locked."?  Google(R)-brand Internet searches don't seem to
show a comprehensive answer.  The possibilities that I gather are
here.  Am I wrong about any of these?  Am I missing any?

- Someone has done a "p4 lock" on the files.  This can be determined
by "p4 opened -a //depot/some/path/...".

- The files have been locked very briefly because it's in the middle
of a submit?

- The files are in a long-term lock because a submit was started
during which the network went down, so now they're stuck locked?

- The protect table doesn't give some sort of permission?

- The user doesn't have some filesystem permission?  They don't have
write permission, either because the file has write permission off, or
(on some systems) the file has been opened by a running process?

-- 
Tim McDaniel, [email protected]
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