Fault Masking is an occurrence, in which one defect prevents the detection of another, there are two perspectives to understand the fault masking.
A Show-stopper defects simply clarify the fault masking, if you test a Login form consist from two data fields, "Login" and "Cancel" buttons, and "Remember me" check box, when press "Login", an unhandled exception fires, so if the "Remember me" check box didn't work you will never know until a successful Login process has been done.
Technically It’s a used technique (by developers) to prevent error result from appearing.
For example, if the “Negative Value” cause a system unhandled exception to fire, the developer may prevent the negative values input instead of handle it, explicitly it solve the issue and prevent another defect from appearing but implicitly if the values entered manually in the database and the system goes to retrieve it, an unhandled exception may fires.
So, as a test engineer you can uncover it by using the Black/White Box techniques by enter the data manually in the database and try to retrieve it by the application.
LinkedIN : Samer Desouky - 20, Aug 2013