Revealing mistake: When the Prince and Princess are catapulted over the wall of the Giant's castle, you can see their shadows on the "sky" above the wall; the sky is a studio wall painted blue.
Revealing mistake: As Jack and the butcher are standing with their backs to the camera and comparing the size of their shadows that stretch out in front of them, Jack's shadow suddenly appears very large. It is understood that the giant is standing behind him. The giant's shadow is quite dark and was obviously added as a special effect, but even though the shadow appears in front of Jack, there is no shadow on Jack's backside, as there would be if the giant were really standing behind him. Then when Jack finally turns around and sees the giant, the large shadow has disappeared.
Continuity mistake: When Jack is speaking to Princess Eloise near the water well, he turns his head to the right. In the following shot, his head is turned to the left.
Character mistake: When Jack offers to sell his cow to the butcher, the butcher says "There isn't much meat on him." Jack points out that the cow is a "her", but after the sale, he says to the cow, "Be a good boy."