From http://article.gmane.org/gmane.mail.imap.cyrus/18840

We have also seen skiplist corruption in seen databases.  I don't have a 
recovery tool, but I have been able to manually recover seen db's to the 
point of corruption so that at least most of the users mails are in the 
correct 'read' state.  Typically, you will see errors like:

DBERROR: skiplist recovery /usr/local/imap/user/k/kdelaney.seen: 0D2C 
should be ADD or DELETE

If you truncate the file at this point, it should fix the problem, and 
the users mail read state will be valid upto the point of corruption. 
To do this, convert the hex to decimal (above would be 1372) and use the 
dd command:

dd if=kdelaney.seen of=kdelaney.seen.fixed bs=1 count=1372

replace the corrupted .seen file with the fixed one and have user log in 
and should be ok.

Seems to work on the couple I have tried it on.