Has there ever been a person who has been canonized as a saint, but later had their sainthood revoked because irrefutable evidence came to light that the person was not the upstanding Christian that everyone thought (perhaps they were an unbeliever or a heretic) or the miracles that were associated with the saint's canonization were determined to be faked?
There are early saints that have been recognized as likely "pious fables" and therefore removed from the liturgical calendar, but none that have been declared to be outright falsehoods. In the modern times, the Church is much better about authenticating miracles.
Being a saint simply means existing in Heaven with God. There are certainly many people that have achieved sainthood who are not formally canonized by the Church. Similarly, even if the life events or miracles attributed to an early saint were exaggerated, that does not mean that person is not in Heaven with God, ergo, a saint.