This article is written from a Real World point of view.
The Animorphs book series contained several continuity errors, or KAASUs (Katherine Alice Applegate Screws Up) as the fans called it. While there are many discrepancies between the books and the TV show, these are not considered continuity errors because of the different medium.
In Book #2: The Visitor, Cassie says that Rachel, after acquiring her cat morph, has four morphs, which is more than any of the other Animorphs. But Jake, at this point, also has four morphs: dog, tiger, anole lizard, and peregrine falcon.
In Book #3: The Encounter, Tobias states that Marco had previously acquired a bald eagle morph, and that he used it during the escape from the Yeerk Truck ship. Marco never acquired a bald eagle; his bird of prey morph is an osprey.
In Book #8: The Alien, Ax says the Hork-Bajir are hardwired to war with each other every 62 years. This is not true in later books as it mentions Hork-Bajir never knew the concept of violence and fighting until the Yeerks and Andalites came (Although it may be that this was part of a propaganda campaign put out after the Andalites failed to save the Hork-Bajir to give the impression that they weren't that significant a loss).
In Book #46: The Deception, Ax, while morphing a human, says one heart disappears and the other becomes a human heart; Andalites have 3 hearts.
In Book #52: The Sacrifice, Timmy is erroneously referred to as Tuan. The character was originally named Tuan; however, Applegate changed it to Timmy before #50 was published. Due to an editorial oversight, the name Tuan still appeared in #52.