IRS matches the SSN with the first four letters of the last name.  For many years, they only did this for the primary SSN on a return.  What happened with your widow client was that she never told Social Security her name had changed from Smith to Jones, when she got married.  And she never filed a return with her name and SSN as primary.  When she filed single as Jones, her SSN was still under Smith.  But in the process of claiming survivor benefits under her married name, Social Security changed their records, and IRS found out before they tried it again.