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 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: 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 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.