garbage in CNFS overview ??

[sysadm at CHS.twbbs.org]

>}       i wonder why they were been created, and how to purge them using
>} utilities?
>
>I personally was told this by someone some while ago
>and this was due to corrupted overview index.  You
>need to recreate all overview and purge those directories
>manually.
>--
>Katsuhiro Kondou  

yeah, i have purge those directories manually before, 
but it seem that it will generate the garbage every day after running 
"news.daily expireover lowmark".
 any other suggestion?