There is some error reported when I run Disk Utility and verify the root volume on my OS X MacBook. So I boot and CMD-S into the shell mode and run /sbin/fsck -fy.

Errors are like:

** Checking catalog file.
   Missing thread record (id = ...)
   In correct number of thread records

** Checking catalog hierarchy.
   Invalid volume file count
   (It should be ... instead of ...)

** Repairing Volume
   Missing directory record (id = ...)

I'd like to know what is the cause of the above errors? Hopefully I will be more careful in the future to prevent them from happening again.

p.s. I am using a SSD and so I assume mechanical hard disk error is less likely. Thanks!