[Dovecot] Corrupted index cache files

[Mike Abbott]

> One or more users?

Many different users.

> Post your dovecot -n output?

Here's some of it.  Not very enlightening.
# 1.1.2: /etc/dovecot/dovecot.conf
base_dir: /var/run/dovecot/
verbose_proctitle: yes
first_valid_uid: 6
last_valid_uid: 50000
first_valid_gid: 6
last_valid_gid: 50000
mail_access_groups: mail
mail_privileged_group: mail
mail_location: maildir:~/mail

> Can you reproduce these easily with my imaptest tool? http://imapwiki.org/ImapTest

Some of them.  When running imaptest I see these dovecot errors:
Corrupted index cache file %s: record continues outside its allocated  
size
Corrupted index cache file %s: record points outside file
Corrupted index cache file %s: used_file_size too large

Plus I see LOTS of errors from imaptest itself:
Error: UIVALIDITY changed: %d -> %d
Error: user%d[%d]: <...>: Header CC changed
Error: user%d[%d]: <...>: Header DELIVERED-TO changed
Error: user%d[%d]: <...>: Header FROM changed
Error: user%d[%d]: <...>: Header IN-REPLY-TO changed
Error: user%d[%d]: <...>: Header MESSAGE-ID changed
Error: user%d[%d]: <...>: Header REFERENCES changed
Error: user%d[%d]: <...>: Header SUBJECT changed
Error: user%d[%d]: <...>: Header SUBJECT changed
Error: user%d[%d]: <...>: Header TO changed
Error: user%d[%d]: UID %d changed Message-Id
Error: user%d[%d]: UID=%d INTERNALDATE changed
Error: user%d[%d]: uid=%d <...>: BODY changed
Error: user%d[%d]: uid=%d <...>: BODYSTRUCTURE changed
Error: user%d[%d]: uid=%d <...>: BODY[%d] size changed
Error: user%d[%d]: uid=%d <...>: BODY[HEADER] size changed
Error: user%d[%d]: uid=%d <...>: BODY[TEXT] size changed
Error: user%d[%d]: uid=%d <...>: BODY[] size changed
Error: user%d[%d]: uid=%d <...>: ENVELOPE changed
Error: user%d[%d]: uid=%d <...>: RFC822.SIZE size changed

> One problem with HFS+ is that hard links are more or less buggy. But  
> v1.1's default settings should include dotlock_use_excl=yes. You  
> maybe should set maildir_copy_with_hardlinks=no, but that shouldn't  
> cause this bug.

Changing maildir_copy_with_hardlinks makes no difference.

You say that the dovecot errors are harmless because dovecot fixes  
them, but still there must be some downside, if only a performance  
hit.  Any other info I can provide to help you figure this out?