"I came in illegal, but I am too an illegal in Mexico because I do not exist on government records there to prove that I am a Mexican citizen here."

http://www.migrationinformation.org/USfocus/display.cfm?ID=888

The lack of an official proof of identity and citizenship means no access to education and health care for children; no opportunities for adults in the formal labor market; no access to microcredit loans; no right to vote; and no human rights. Moreover, the “officially invisible” are more attractive targets for human traffickers, illegal adoption rings, and other organized criminals in Mexico. Thousands of births in Mexico continue to go unregistered each year, and as these disenfranchised people with no official identities make their way across the Mexico-US border, the problem of doubly-undocumented immigrants in the United States will grow larger still.