Clash of the Titans (1981)

Revealing mistake: When Calibos is speaking to Thetis' statue, as he screams "I demand justice", you can see the left horn on his head wiggling around, revealing a flimsy prop.

Revealing mistake: During the scene when Argos is being destroyed, a citizen falls backward due to the rushing waves and you can see a pair of black running shorts with white trim under his tunic.

Revealing mistake: At the beginning when the ruler is making his speech, you can see his modern day fillings in his upper molars.

Revealing mistake: When the ruler of Argos is cursing his daughter prior to setting her adrift, we are treated to a couple of close ups where his modern dental fillings are beautifully displayed.