At first there doesn't seem to be any particular reason why Arbron picks a yellow VW Bug to convey Jake down to meet the Free Taxxons. But then you remember that when Arbron first became a Taxxon nothlit, the Living Hive brought Elfangor down to him in a yellow Mustang. Knowing Arbron's humor, he probably enjoyed the symmetry.
One of the things Ax finds amusing about humans think the Internet is more advanced than the telephone. At first, this seems equally silly to the reader, but then you realize that Andalite telephones have to transmit Telepathy, not sound.
In The Alien, Ax fixes Marco's dad's programming. This makes little sense at fist, since human computers should be completely different than Andalite computers... until you realize that Elfangor exchanged ideas with 'Bill' and 'Steve'. And even said he had to use simple terms like "WINDOW" to explain advanced concepts!
This is pure Death of the Author stuff, but remember that "Andalite traitors in the ranks" plot that Applegate just kinda forgot about? Well, the last time it comes up is during the Leeran campaign. Leerans have telepathic powers. Purging your military is very easy when you just allied with a people who can instantly sort out traitors.
In Visser, it's said that Hork-Bajir are a Class Three species because they are not numerous enough and they can't be quickly bred. That means they breed the creatures they take as hosts. Obviously, this is not explored in the series, but in the books where the horrible future of Yeerks completely taking over humans is shown, this means that aliens in those humans' brains are making them have sex with each other so those aliens can conquer the galaxy. Want to take the horror a step further? Yeerks have really poor senses in their natural bodies and get drunk on the senses of their hosts, they explore them and indulge them for pleasure. Think about how eager just normal people tend to be regarding having sex, because it feels so good, now add in a sensation starved parasite in charge of the decision making process and thousands of others in the exact same situation for them to experiment with. And the host has absolutely no say in it whatsoever but still has to experience it. You'd be getting raped until the Yeerk gets bored of it, if ever, and possibly they may not care about the age of the hosts involved.
In The Invasion, Jake nearly tells Tom that he's the tiger when they're in the Yeerk Pool. As Tom is reinfested shortly afterward, Jake nearly sold out all of the Animorphs on day one.
David's fate - rats only have a lifespan of two to three years. So instead of a death sentence, the Animorphs just gave him a slightly prolonged death sentence.
When Jake's Yeerk starts dying, they both get treated to our first look at Crayak. Does it happen to every Yeerk Controller? And if so, is that why many of the humans who escape that control wind up crazy?
Adult Fear: Much of the Nightmare Fuel of the series can go over the heads of its target age group, who are more likely to focus on the cool adventures and animals. However, if you're an adult who decides to do a nostalgic re-read (this troper did), not only do you catch everything this time around, but it's compounded by the fact that all of it is happening to young children.