Continuity mistake: When they arrive on the island of Talos, the bronze giant, Hercules and Hylus find the statues atop huge plinths. In close up, they are looking up at the statue of Talos, then in a wide shot they start to walk towards it across bare rocks, then in close up they are back where they were with plants behind them.
Continuity mistake: In the scene where the two men are carrying the coffin with the cross on it down to the crypt, the coffin is being held with the cross pointing towards the last man. But in the next shot the cross is pointing towards the first man, as the coffin has been turned around. Then in the next shot the coffin is in its original position, with the cross pointing towards the last man. There is not enough time for the coffin to rotate this many times between the two men carrying it.
Audio problem: While Marcello Mastroianni is waiting at the station and Sandra Milo's train can be heard arriving off-screen, we hear much whistling by a steam train, but when it finally appears, the train is being hauled by a diesel locomotive. [As a side note/explanation - all sound in Italian movies, including the dialogue, is recorded afterwards, sometimes by different actors, eg. Claudia Cardinale in real life has a regional accent.]