The secret U.S. government no-fly list of people banned from flying has jumped from 10,000 to 21,000 according to the Terrorist Screening Center that is responsible for maintaining the list. People on the list are regarded as actual or suspected terrorists. The list includes persons such as Maher Arar who was cleared of any terrorist connections ages ago and awarded 10 million dollars in damages for his treatment in Syrian jails where he was tortured after being rendered there by the U.S.
The threshold for being put on the list has been lowered. Formerly one had to be considered a threat to aviation but now anyone who is considered a threat to domestic or international security can be put on the list. About 500 U.S. nationals are on the list. The ACLU says: "It's a secret list, and the government puts people on it without any explanation. Citizens have been stranded abroad."
However, an official claimed that the list was under continuous review to ascertain that the right people are on it and that those who should not be on the list are removed. Many terror suspects travel under names that may be quite common so that people with the same name often find themselves delayed or denied the right to board a plane. It may be difficult if not impossible to have ones name removed from the list. For more see this article.