Frequent 'Filesystem Corrupt', Invalid File count

I have been getting this message every a couple of days in my MBP or iMac running 10.6.2. It happens/visible only when I runs Disk Utility and Verify Disk. Of course, this brings unsettling point to me when it comes now and then. Not to mention I have to boot from DVD to run disk utility to fix this

Error Message: Invalid file count (It should be xxxxx instead of xxx+1) AND
Error Message: Invalid volume directory count (It should be xxxxx instead of xxxx-1)

Any idea what causes this and what's the impact i.e. do I lose any files?

Thanks!!