Factual error: When Carl first receives 26 at the hatchery, he asks what kind of dinosaur she is. He is told she is a Hadrosaur (duck-billed dinosaur), but when we see her, she is some sort of Ceratopsian (i.e.: Triceratops). She is later referred to as a Chasmosaur (which while not looking like 26, IS a Ceratopsian).
Plot hole: Although the errand bird always insists on the postage and even asks for additional fees for more than twenty words, he never gets any payment from the assigning characters.
Plot hole: The dinotopian island is so large that there are mountain ranges, massive rivers, and whole towns so far apart that it takes days to travel between them. Yet this island is also so close to the mainland that they stumble upon it with a single engine propeller driven airplane. Such a land mass would be just a little hard to miss.
Other mistake: When Carl and David go down in the submarine, they come across the sunken plane that they crashed in. As they approach the plane, it shows that the it is clearly on the right side of the sub as they go from the tail to the prop. When it cuts back to them in the sub, they are looking to the left. A cut back to the plane still shows that it could only be on the right. Carl then looks out the left port hole of the sub and it clearly shows them going by the plane from prop to tail.