Intermittent make_bak_dir mkdir "<path>" failed: File exists (17)

[Charles]

Rare and intermittent message groups suggest a race condition ... ?

Here's the pattern of messages:

rsync: make_bak_dir mkdir <path> failed: File exists (17)
rsync: keep_backup failed: <path> -> <path>: No such file or directory (2)
rsync: stat <path> failed: No such file or directory (2)

How rarely?  We run a scripted rsync daily on ~50 personal computers and
servers, typically backing up /etc, /home, /root and /var.  These
message groups appear about once a day.  We have not seen the group of
messages more than once per rsync run.

How intermittently?  We have gone 3 days with no such messages and at
worst have seen them from 3 rsync runs (on different computers) in a day.

The rsync options generated by the script are:

rsync --backup --backup-dir=<backup dir> --delete-during --devices
--exclude=<backup dir> --group --links --owner --perms --timeout=300
--recursive --specials --times --verbose

Here's a log excerpt (the first line is from the script; the rest from
rsysnc):

12:31:59 rsync_bu.sh: ERROR: rsync return code 23. Output:
sending incremental file list
lib/alsa/asound.state
lib/apt/lists/
rsync: make_bak_dir mkdir "/mnt/backup/rsync/var/_Changed and deleted
files/2014/Mar/03 at 12:31" failed: File exists (17)
rsync: keep_backup failed:
"/mnt/backup/rsync/var/lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_precise-proposed_Release.gpg"
-> "_Changed and deleted
files/2014/Mar/03 at 12:31/lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_precise-proposed_Release.gpg":
No such file or directory (2)
rsync: stat "/mnt/backup/rsync/var/_Changed and deleted
files/2014/Mar/03 at 12:31/lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_precise-proposed_Release.gpg"
failed: No such file or directory (2)
deleting
lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_precise-proposed_Release.gpg
[snip]

The message groups seldom appear so close to the beginning of the log.

Versions:
* rsync: 3.0.7 and 3.0.9
* OS: Debian 6 and 7, Ubuntu 12.04