The prop of the dead Megaloceros looks drastically different from the computer generated version. Further, after it dies, drops of blood appear around its mouth between shots.
When it chases down the Neanderthal, the woolly rhino has brown hair when it's a CGI model, but when it's a physical prop, its hair is more reddish.
In the opening scene, when the mammoth herd begins to walk away from their trapped sister, they cast no shadows, but in the next shot there's long shadows on the ground.
The cave lion is just a reused and slightly edited computer model of a saber-tooth cat, and thus it looks nothing like a lion. It has the large front legs, sloping back and characteristic head shape of saber-tooth cats, and it even has fangs sticking out of its mouth, unlike a real lion.
In one shot, when a male mammoth bellows at two cave lions eating a human corpse, its trunk sinks into and passes through one of its tusks. This error is fairly noticeable, as it happens in the middle of the screen and it curiously lasts for several seconds.
The antlers of the two fighting Megaloceros pass completely through each other.
During the Neanderthal attack on the mammoth herd, a female Neanderthal can be heard screaming "Come on!" in modern English twice.
Even though the faces of the actors playing the Neanderthals were carefully masked, their arms remained nice and shiny, as seen in certain shots.
When the mammoths bath themselves in mud, none of the computer-generated animals seem to be covered with the stuff. They remain perfectly clean, and in one shot, the mud that they spray through their trunks actually passes through their bodies. Also, several drops of mud simply vanish midair, like in cartoons.
When the episode title card appears, as the herd leaves the mammoth stuck in ice behind, there is an odd area around the trapped mammoth where the shadows cast by the herd are "cut off". Also, the parts of the baby mammoth that are in this area lack hair, but as it moves out of the area, its hair reappears. There is likely a strange CGI compositing "artifact" on the screen that somehow blocked out the rendering of shadows and hair in the area.
As the herd begins their journey south, in one shot the baby mammoth walks over a dried-up bush. But instead of flattening it, the bush simply disappears behind the mammoth as it walks near it, then as it walks over it, the bush gradually "fades" back into existence.
When the rocks are hurtling toward the camera, the hand of a crew member can be seen in the upper left corner of the screen.