Revealing mistake: When the parted Red Sea comes together, if you look behind Pharaoh as his troops are being engulfed by the water, notice in the background that there are two boulders floating in the water, not being pushed, but floating lightly.
Revealing mistake: In the scene where Moses takes the first journey up Mt. Sinai for his very first view of the burning bush, the walking scene shot has Charlton Heston following the camera as the camera dolly's backwards towards the bush. As Heston and the camera movement comes to a halt, you can see two long tire tracks in the sand created by the dolly truck that housed the camera and cameraman.
Revealing mistake: All of the women in the film have 1950's-style haircuts.
Revealing mistake: When Rameses is casting Moses out into the desert, he stands in his chariot and delivers his speech about ruling over the scorpions. The pyramids behind him are 20th century photos, i.e. they look 4000 years old. In the time of Rameses, they would have been substantially newer - 1000-1500 years old.
Continuity mistake: When Moses confronts his Egyptian mother Bithiah with the Hebrew piece of cloth, she is beading on an Egyptian loom. During this scene, the string that she is beading (the farthest to our right) first matches the height of the other strings. One shot later, there is suddenly more beads, making the string higher than the others. (00:57:40)
Other mistake: When Moses' wife Sephora is out guarding the sheep, Moses is with his son at a tent. Sephora calls him because there is a stranger behind the rocks. When she calls you can hear that the sound is indoor-sound from a set, not outdoors. This continues when Moses calls, the dogs bark and so on.
Audio problem: After the Red Sea parts, and almost all of the Hebrews are across the dry expanse, the pillar of fire holding Pharaoh back disappears, and the chariots head into the expanse themselves to pursue the Hebrews. A very young Hebrew boy at the rear of the masses says 'Look, Grandpa! Pharaoh and his men are coming!' and points at them with his free hand. Watch his mouth: the child is merely moving his lower jaw up and down. Apparently the child actor was too young to learn any lines so the producers improvised.