The narrator claims that the Sauropods went into decline during the Jurassic, but that is not exactly true. While they received competition from other dinosaurs in the Northern Hemisphere, they were still very successful overall, especially in the Southern Hemisphere, and that was when their largest species evolved.
The narrator states that Diplodocus can grow to over 40 m long. In reality, Diplodocus barely reached 30 m. Scientists now believe that a similar dinosaur, Seismosaurus was actually a larger species of Diplodocus, but even then, the animal wouldn't have grown so big.
The crest of the Allosaurus is shown directly above its eyes, but in reality, they were situated lower, in front of the eyes, and they had a pair of smaller crests running down their snout. This was corrected in later follow-ups of the program, but it's an error here.
During the attack scene at the end of the episode, the flesh on the side of the Allosaurus' tail moves in unison with its leg. While in reality, the thigh muscles of these animals probably did connect to the tail, the animations shows it shifting position unrealistically, as if the muscle and skin detached from the tail bones.
Sometimes when the Diplodocus open their mouths, their upper teeth "stretch" down below the line of the upper lip. When their jaw is closed, the teeth "shrink" back to being normal-length. It seems the jaw-moving animation somehow got carried over to some of their teeth. This error also appears in the special Allosaurus: A Walking with Dinosaurs Special.
The leaves and branches disturbed by the Diplodocus hatchlings, as they make their way into the woods, are not actually under their feet but several feet away from them - they are apparently disturbed by nothing, as if the computer generated animals had been animated in the wrong place.
When the baby Diplodocus are inspecting the skeleton, there is a line on the top of the screen where the background is visible through the foreground objects, revealing that they've been added into the shot digitally and cropped incorrectly. This, as well as most other image cropping errors, was fixed on later DVD releases.
When the surviving Diplodocus reach the plains after the forest fire, two Allosaurus are resting nearby, but they don't affect the plants under their heads and their shadows are shaped as if the ground under them were flat.
The show's Diplodocus models suffer from noticeable clipping errors around the "seams" where the limbs attach to their body. These are very noticeable on their shoulders, which can be seen in a number of close shots. This is likely due to the show's primitive CGI effects, as the animals were not composed of a singular 3D computer model, but were created via "sticking together" separate body and limb pieces.