As I learned watching the TV show Lovejoy, a number of great painters started out as forgers, so who knows what may be a forgery that is accepted as the real thing.
I was also shocked to learn how many painting attributed to a famous artist were actually done by their students. Also the authentication process is very subjective. Like when a historian says things like “the boldness of the brush strokes” I also remind myself that most art value is derived from financial speculation and money laundering.