Typically happens when a child is born in the US to non-US citizens who return to their home country. Often they do not realize that simply being born in the US creates citizenship (many countries do not have birth citizenship). There have been court cases in the US when a family comes to the US on vacation and one of the parents gets arrested for failure to file tax returns - the birth happened in the US and the hospital filed the paperwork and the computers remember.

As others have mentioned, it isn't just a case of filing IRS paperwork. I know many non-US banks will refuse to do business with someone if they discover the person is a US citizen because the bank paperwork is not worth it.