It is always possible for a famous artist's previously unknown work to be found hidden away somewhere. It would nevertheless be authentic, and I think even an expert might be forgiven for erring on the side of caution. Such an error would however decrease your credibility as an expert considerably.
People who are considered experts would have studied and would know intimately an artist's body of work. There are some extremely talented forgers in the world, but I would say that to label even a really excellent fake authentic would be the more serious transgression, particularly to the party who may have paid big bucks for it.
