If you're dead. There might be other cases too, but if you're no longer alive you're not a citizen, at least in the U.S. This is the only instance of this situation I know of.

Certainly it is. Especially in countries where where there are civil wars and attempts to overthrow the government on a regular basis. This may cause situations where people live in exile in exchange for their family not being killed by the new governing organization. So they go to another country but they cannot get citizenship because if their association with their former hostile government. Also there is an Iranian guy by the name of Merhan Nasseri who is stuck in a French airport. The government kicked him out of the country so he decided to move to europe. He was on his way to England when his passport, luggage, and immigration paperwork was stolen at a train station in France. After one of his relatives wired him a plane ticket to London, the authorities deported him back to the french airport because he had no passport. And for the same reason, he was not allowed to enter France past the borders of their international airport. Since then he was allowed to leave the airport however he has decided that he didnt want to leave. Thus making him a citizen of nowhere. More information on this story can be found at http://www.straightdope.com/classics/a990820.html

Yes, in the USA the 14th amendment can be enforced at any time. Babies born to illegal aliens could be and might be denied citizenship and have no birthright at all. The law will state "exactly" that. The babies through no fault of their own will not be able to claim a country of birth. The US is now claiming that these children grow up and sponsor their parents to the US. 14 th Amendment’s ratification makes clear that the Citizenship Clause’s proponents were careful to preclude any automatic grant of citizenship based only on birth within the territory of the United States. "All persons born or naturalized in the United States, and “subject to the jurisdiction thereof," are citizens of the United States and of the State wherein they reside." “subject to the jurisdiction thereof,"

I believe homeless people, who are mobile, would fit into this category. i have met many people that have no home. i asked one homeless person, what is your address? his answer was, i don't have an address. at the least, most people have to have a p.o. box number, to receive governmental assistance. a mobile, homeless person is a citizen, from absolutely nowhere. sorry situation.