"In the Time of Dinosaurs" describes Spinosaurus as having shorter hindlegs than a Tyrannosaurus, before discoveries in 2014 suggested this may have been true.
The idea that the Animorphs would always morph an animal of the age of the acquired one was never explained, and used as a plot device so that they could look exactly like controllers who they acquired and impersonated. However, years after the books were published, the discovery of epigenetics, the way organisms' DNA changes while they are still alive, show that the DNA reflecting how old an acquired animal is was very accurate.
Author Appeal: K.A's favorite things - Star Trek, the Rolling Stones, Dr. Pepper, Mustangs, etc - are all prominently featured. Similarly, her personal views creep often into her work.
Author Tract: A handful of the books exist for no other reason than to promote the views of K.A. (and later, her ghostwriters). #04 The Message (whales are sentient), #09 The Secret (logging is bad), and #28 The Experiment (eating meat is very bad) are the worst offenders. A more mild offender is #16 The Warning (the internet is weird), which is redeemed by strong characterization and a truly awesome villain.
Fan Nickname: Emohawk for Tobias. Hawkward for Rachel/Tobias moments. Visser Mom for Edriss/Eva. KASU for "Katherine Applegate Screws Up/Screw-Ups", or the mistakes between books by the author and her ghostwriters. Applegrant for the Applegate/Grant co-author duo.
Older Than They Think: Watch Star Trek II: The Wrath of Khan, from 1982. Do those slug brain parasites - that crawl into your ear and take over your mind - seem a little familiar? Considering that Applegate is a known Trekkie, this is almost certainly not an accident.
Promoted Fanboy: Erek King, the fan who won a contest to have a character named after himself. The Erek King character ended up becoming one of the most important secondary characters in the series.
Referenced by...: An instance of SCP-1468 started producing a carving of The Invasion before dying from malnutrition.
Tobias breaks the fourth wall at the end of In the Time of Dinosaurs to acknowledge that scientists dispute that several of the dinosaurs depicted in the book were alive at the end of the Cretaceous, asking the reader, who're you gonna believe, some scientists with nothing but bones, or a guy who was actually there? Although it should be noted the Deinonychus are portrayed as featherless.
The pack mentality of wolves mentioned in the books has since been discarded by science along with the biology and mentality of many other creatures featured since they were written.
Applegate and Grant based most of the characters on people they knew. Applegate has said that the character Loren is based on herself. Grant is described as a lot like Marco but it's unknown if this was intentional (probably more of an author projection thing seeing as Marco's got an Expy in Grant's series).
Applegate also said Cassie was most like her, while Marco was most like Michael Grant.
Acting for Thousands: Because only one Hork-Bajir costume existed, a single Hork-Bajir was the only representation of an entire race. Still a better deal than the Taxxons got; they were cut from the show altogether.
Dawson Casting: A mild example, as the age of the book characters had not yet been revealed when Ani-TV went into production.
Keep Circulating the Tapes: Only had 12 episodes released on VHS, and nothing else. Fans have taken to uploading the rest of the series online, even though most consider it a bad show. Qubo (one of Ion's spinoff cable channels) reran it in 2014.
It's also very obvious that there was only one Hork-Bajir costume, and the Taxxons never appear as they would clearly be impossible on the show's budget.
Recursive Adaptation: Meet the Stars of Animorphs, a companion book released shortly after Ani-TV started airing.
The Other Darrin: Tom Barnett replaces Eugene Lipinski as Visser Three in Season 2. Given that this is a series where people can change into anything alive (within reason), it is more easily explained then other examples of the trope.
Button Mashing: The combat sections of Know the Secret basically amount to this. There's a kind of Combo system to it, but it's very rudimentary.
Executive Meddling: Applegate and Scholastic got caught up in the post-Columbine hysteria over violence in video games and had SingleTrac revamp Shattered Reality, which was at that point already a Troubled Production.
Filler Villain: Mostly seen in Shattered Reality. In addition to renditions of the baddie aliens, new enemies were created, some of which make more sense than others. The crowning moment of absurdity comes when you end up fighting pterodactyls in the Gardens level.
Polygon Ceiling: In Know the Secret. It gives the game a really dated look, which is a shame, because it's quite serviceable in other respects.
Save-Game Limits: There is no regular save feature on the GBC game. Think about that... a game clearly modeled after Pokémon, with the same basic menu screen and gameplay mechanics has no save feature. Instead, either by pressing Select or reaching a checkpoint you be given a password that you have to write down that will return you to something approaching your current party next time you start the game. This means that unless you are either using an Emulator with savestates or trying to marathon the whole damn thingin one sitting (which is theoretically possible, thanks to unlimited continues), it's virtually impossible to beat the game in a way that feels like a continuous, complete narrative.