> How did I detected the database was corrupted?

Yes. How ?

> First one of the users got an error doing something with my application.

What error ?

Did you look at firebird.log ?

> What do you mean saying "wrong usage of gfix". The first thing I did was to
> do gfix -mend. Was this wrong?

Yes it si wrong. The first thing you must to do when you have suspect database
is to stop the server and make a file copy of database. The second is to restart
server and run gfix -validate -full. Next action is depends on results of previous ones.

Regards,
Vlad