Here's an assertion failure that just happened. Unfortunately, I don't have a core file so this may be useless...

This is with CVS version from 2005-11-03.

It looks like he expunges all of his mail.

dovecot: Nov 21 17:37:20 Error: 13261 imap(username): mbox sync: Expunged message reappeared in mailbox /mailhome/new/b/n/username/mbox (UID 1008 < 1606, seq=816, idx_msgs=1238)

dovecot: Nov 21 17:37:23 Error: 13261 imap(username): file mail-index-transaction.c: line 129 (mail_index_buffer_convert_to_uids): assertion failed: (*seq != 0)

dovecot: Nov 21 17:37:24 Error: child 13261 (imap) killed with signal 6