Corrupted transaction log and redownloading

Hi,

today we had to hard-reboot one of our dovecot proxy backends and
after boot up, lot's of accounts were showing this error and appeard
empty: Error: Corrupted transaction log file ...

After deleting the indexes (not uid file, problem wasn't in deleting
indexes, i'm doing this often), e-mails showed up ok but e-mail
clinets started to download them again.

Why this happened and how to prevent it (other than 'not
hard-rebooting')? I know that reboot could cause corruption of uid
files in some cases but this time it looks like this was caused by
corrupted indexes - how index corruption can cause redownloading and
why?

Thanks for any info.

azur