Hermes and the infant Dionysus (ca 340 BC).
This is one of the most famous statues from antiquity, attributed to the famous Athenian Praxiteles (born ca 395 BC) by Pausanias. It was considered a Greek original from the moment it was found in 1877, but later re-examined by Carl Blümmel (1927), who concluded that it was a Roman copy. It might be added that Blümmel, who was an expert on classical history and a sculptor himself, based his argumentation on eight technical points such as cutting technique, stature and the drilling.
The debate is by no means over today, Olympia still claim that the piece is an original, but common sense should convince anyone that this is a early imperial copy.