Continuity mistake: When they are fighting the giant scorpions, there are three distinct scorpions fighting. Each of the remaining men faces a scorpion 1 on 1. One of the men dies fighting, and thats the last we see of that scorpion. Thello kills his scorpion, then throws Perseus his sword who then kills his scorpion. The third one just magically disappears.
Continuity mistake: In the beginning scene, the ruler of Argos is condemning his daughter and son to the sea with a group of guards present at the shore. While he is giving his speech, there is a brief moment when the whole group can be seen getting drenched by a wave crashing ashore, but in the very next moment of his speech - even up to the daughter's cast off - no one is wet.
Continuity mistake: When it shows the man being burned at the stake in the city, the man is just a wooden carving. As brief as the shots of it are, it's extremely obvious, particularly if you use slow motion. (The use of slow motion is permitted when it reveals things such this.)
Continuity mistake: When Calibos gives Andromeda the necklace, he drapes it around her neck (unable to clasp it with his long fake nails). In the next and following shots, the necklace is clasped.
Continuity mistake: When Perseus is taming Pegasus, a claymation model is used for the long shots of the famous flying horse. However, when it shows Perseus jumping on the back of the real horse, you can clearly see that the horse has no wings. They couldn't figure out a way to strap on fake wings for the close-up?
Continuity mistake: After getting Pegasus back, Perseus hands the horse's rope lead to Ammon. In the following claymation shot, the rope has disappeared.
Continuity mistake: During the riddle challenge, Andromeda's hair is in a stylish updo while standing at the altar with her mother Cassiopeia. Perseus enters moments later and solves the riddle. But in the next shot as they enter the courtyard together, Andromeda's hair is suddenly down around her shoulders and she looks like a little girl, instead of the mature woman she was at the altar moments before.
Continuity mistake: When Perseus and the other soldiers are on Charon's boat, you see a couple of close-up shots of Charon's bony hands on the oar. However, in the long shots when Perseus is looking back at him, you can see that he has regular hands wearing gloves.
Continuity mistake: After Perseus kills Medusa and her headless body falls, blood starts to pour out of her neck in a small pool. The camera instantly cuts to his shield, and it is already sitting in a very large pool of blood, and is starting to dissolve. There was no time lapse, so there was no time for the pool of blood to have expanded that far.