Many are from countries considered a threat to the United States Of America.

An audit of the Department Of Homeland Security shows that at least 858 immigrants were mistakenly granted citizenship. According to a story at KVIA.com:

The report does not identify any of the immigrants by name, but Inspector General John Roth's auditors said they were all from "special interest countries" - those that present a national security concern for the United States - or neighboring countries with high rates of immigration fraud. The report did not identify those countries.

The KVIA story goes on to say:

Mistakenly awarding citizenship to someone ordered deported can have serious consequences because U.S. citizens can typically apply for and receive security clearances or take security-sensitive jobs.

The report goes on to say that fingerprints are missing from federal databases for as many as 315,000 immigrants with final deportation orders or who are fugitive criminals.

Some past "accidental citizens" were able to get aviation or transportation worker credentials, giving them access to the secured areas airports and other sensitive places. Another managed to become a cop.