[Dovecot] unwanted maildir directory

[Mike Abbott]

Dovecot-2.0.3 reported:

Error: Maildir: Found unwanted directory /path/to/mail/user/cur/:2,FST, but rmdir() failed: Directory not empty

and sure enough, this directory really does exist and contain a valid message file:

# cd /path/to/mail/user/cur/
# ls -lR
total 384
drwx------   8 vmail  vmail    272 Sep 24 09:06 .
drwx------  13 vmail  vmail    442 Sep 24 09:06 ..
-rw-------   5 vmail  vmail  41045 Sep 24 08:44 1285336100.M879758P1166.my.mail.server,S=41045,W=41755:2,RST
-rw-------   5 vmail  vmail  41045 Sep 24 08:44 1285336100.M910424P1166.my.mail.server,S=41045,W=41755:2,RST
-rw-------   5 vmail  vmail  41045 Sep 24 08:44 1285336337.M656599P1166.my.mail.server,S=41045,W=41755:2,ST
-rw-------   5 vmail  vmail  41045 Sep 24 08:44 1285336355.M88818P1164.my.mail.server,S=41045,W=41755:2,FS
-rw-------   1 vmail  vmail  14953 Sep 24 09:00 1285336838.M483116P5720.my.mail.server,S=14953,W=15225:2,
drwx------   3 vmail  vmail    102 Sep 24 09:00 :2,FST

./:2,FST:
total 88
drwx------  3 vmail  vmail    102 Sep 24 09:00 .
drwx------  8 vmail  vmail    272 Sep 24 09:06 ..
-rw-------  5 vmail  vmail  41045 Sep 24 08:44 1285335855.M242128P1165.my.mail.server,S=41045,W=41755
#

Dovecot must have created this non-Maildir++-ish directory, but why, why in cur/ not tmp/, and why didn't it clean it up?  (Not the "Directory not empty" error specifically; I mean in the larger sense.)