test_mbox is an mbox mailbox with a few messages in it.

>>> import mailbox
>>> inbox = mailbox.mbox('test_mbox')
>>> inbox.lock()
>>> inbox.popitem()
(0, <mailbox.mboxMessage instance at 0x7f78016bc680>)
>>> inbox.flush()
>>> inbox.unlock()
>>> inbox.lock()
>>> inbox.popitem()
(1, <mailbox.mboxMessage instance at 0x7f7801653320>)
>>> inbox.flush()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/mailbox.py", line 633, in flush
    (self._file_length, cur_len))
mailbox.ExternalClashError: Size of mailbox file changed (expected 141289, found 141147)

Actually, you don't even need to unlock() and lock() the mailbox  before the second popitem() and flush().

The fix seems to be very simple:

diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index a677729..2be4c83 100644
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -675,6 +675,7 @@ class _singlefileMailbox(Mailbox):
                     new_file.write(buffer)
                 new_toc[key] = (new_start, new_file.tell())
                 self._post_message_hook(new_file)
+            self._file_length = new_file.tell()
         except:
             new_file.close()
             os.remove(new_file.name)

I guess all single-file mailboxes have this issue, not only mbox. I'll still need to add tests when I have time.

As I suspected, all single-file mailboxes(mbox, MMDF, Babyl) have this issue. Attached a patch with tests.

New changeset 0add70dd3c43 by Petri Lehtinen in branch '2.7':
#15036: Make a repeated changes and flushes work with single-file mailboxes
http://hg.python.org/cpython/rev/0add70dd3c43

New changeset 714b8f91f3d4 by Petri Lehtinen in branch '3.2':
#15036: Make a repeated changes and flushes work with single-file mailboxes
http://hg.python.org/cpython/rev/714b8f91f3d4

New changeset 87d119117560 by Petri Lehtinen in branch 'default':
#15036: Make a repeated changes and flushes work with single-file mailboxes
http://hg.python.org/cpython/rev/87d119117560

The news item isn't completely clear.  It sounds like the mailbox is now automatically being flushed between pops, but what you really fixed is popping if the *application* does a flush between them, right?

Yes, this is what I tried to say. It's hard for me to find a good
wording, so what would you suggest?

I also noticed now that there's a typo in the commit messages. But
those cannot be fixed anymore.

"Mailbox no longer throws an error if a flush is done between operations when removing or changing multiple items in mbox, MMDF, or Babyl mailboxes."

New changeset 8b38a81ba3bf by Petri Lehtinen in branch '2.7':
Fix NEWS entry for #15036
http://hg.python.org/cpython/rev/8b38a81ba3bf

New changeset 38e2a87c9051 by Petri Lehtinen in branch '3.2':
Fix NEWS entry for #15036
http://hg.python.org/cpython/rev/38e2a87c9051

New changeset 072b08989731 by Petri Lehtinen in branch 'default':
Fix NEWS entry for #15036
http://hg.python.org/cpython/rev/072b08989731

Perfect, fixed.