Running 2.0.beta5 on Mac OS X with HFS+J, I often see this error in the log:
Fri May 14 12:25:59 gromit dovecot[5466]: imap(pid 8101 user joe): Error: mdbox /var/spool/imap/dovecot/mail/mailboxes/INBOX/Foobar/dbox-Mails: map uidvalidity mismatch (1273852899 vs 1273853949) Fri May 14 12:25:59 gromit dovecot[5466]: imap(pid 8101 user joe): Disconnected: Internal error occurred. Refer to server log for more information. [2010-05-14 12:25:59] bytes=1337/4443 Fri May 14 12:25:59 gromit dovecot[5466]: imap(pid 8101 user joe): Warning: mdbox /var/spool/imap/dovecot/mail/storage: rebuilding indexes
The logs also reveal that this user was logged in twice at the time. There appear to be two issues here: the uidvalidity mismatch, and the path in the "rebuilding indexes" message does not match the path in the "uidvalidity mismatch" error.