[Dovecot] index corruption

[rosenfield at users.sourceforge.net]

Hi people

Just switched to Dovecot yesterday, and I've already got about five
reports of users with duplicate mail in their inbox :-/.  Not as bad
as lost email, mind you, but pretty annoying still.

Checking the server log, it says:

postfix/smtpd: disconnect from unknown
deliver: msync() failed with index file maildir/dovecot.index:
Input/output error
unionfs: new lower inode mtime (bindex=0, name=maildir)
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
IMAP: Disconnected: Internal error occurred. Refer to server log for
more information.

This is dovecot version 1.0.3, the log presented here relates to only
one user; user name snipped for anonymity purposes.  Afterwards things
just get worse and worse with the affected user:

dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
deliver: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: Corrupted transaction log file
maildir/.SPAM/dovecot.index.log: invalid new transaction log sequence
(1 >= 1)
dovecot: IMAP: fscking index file maildir/.SPAM/dovecot.index
dovecot: IMAP: Transaction log file maildir/.SPAM/dovecot.index.log:
marked corrupted
dovecot: IMAP: Corrupted transaction log file
maildir/.SPAM/dovecot.index.log: invalid new transaction log sequence
(2 >= 2)
dovecot: IMAP: fscking index file maildir/.SPAM/dovecot.index
dovecot: IMAP: Corrupted transaction log file
maildir/.Junk/dovecot.index.log: invalid new transaction log sequence
(1 >= 1)
dovecot: IMAP: fscking index file maildir/.Junk/dovecot.index
dovecot: IMAP: Transaction log file maildir/.Junk/dovecot.index.log:
marked corrupted
dovecot: IMAP: Corrupted transaction log file
maildir/.Junk/dovecot.index.log: invalid new transaction log sequence
(2 >= 2)
dovecot: IMAP: fscking index file maildir/.Junk/dovecot.index
deliver: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
deliver: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error
dovecot: IMAP: msync() failed with index file maildir/dovecot.index:
Input/output error

What could be the problem?
How do I go about debugging an issue such as this?