The situations under which I've seen this happen (mainly to others :-)...
*Running Novel networks without latest patches and certain settings
*Having unreliable network connections, especially peer-peer without a real stand-alone server.
*Allowing users to siwtch off
* Win 3.1 :-)

Any ring a bell?

>There is a data file which is about 30MB in size and the index of this file gets corrupted very often.
>
>Do you have any idea of the cause of the above?