Cassie has a book that's one, The Hidden. A random buffalo and an ant somehow acquire morphing powers as the Yeerks cause trouble with Helmacron technology. It's as bad as it sounds.
Book 41: The Familiar is this as it is really weird and often makes no sense, and at the end is revealed to be some kind of dream or illusion caused by some sort of being or beings who are confirmed by Word of God not to appear anywhere else in the series, making it unnecessary to the overall plot.
Megamorphs 2: In The Time of The Dinosaurs: A nuclear sub explodes, tearing a hole in space-time that sends all the Animorphs to the time of the dinosaurs. They meet the Nesk and the Mercora species, getting caught in the war between the two. For reasons never explained, morphing won't heal Tobias's broken wing in this adventure.note Applegate later admitted that she wrote "Tobias breaks his wing" in her summary to her ghostwriter after forgetting that morphing can heal injuries. The ghostwriter thus wrote a scene of Tobias trying to heal it but finding it can't. The K-T extinction comet arrives and kills all the dinos and aliens, as well as rocketing the Animorphs back to their own time. After this adventure, the Animorphs find they can not morph into their acquired dinosaur forms.
The third and fourth Megamorphs books. Time-Travel involving a time machine which may be a piece of the Ellimist, with Nazi who aren't Nazis and Hitler as a random jeep driver, and the It's a Wonderful Plot episode in which the Animorphs never got their powers because Jake accepted the Drode's deal to have never been part of the war (and don't know what's going on until the end).
The Forgotten also counts, for similar reasons. In fact, any book written with the Sario Rip effect is an instant candidate for a Bizarro Episode.
The Attack isn't a Sario Rip book, but it nevertheless doesn't allow the Animorphs to keep the morphs they acquire. The Ellimist transports the six of them plus Erek to a far-off planet to battle a group of Howlers as part of a game against Crayak, and meet the Iskoort, who have Weird Trade Union as their hat. It ends when Jake manages to morph a Howler and upload his memories to their racial memory, destroying their Crayak-enforced innocence and ensuring that the next time they're sent to destroy a race, they'll instead try to kiss everything in sight rather than killing it. Though this one is rather more fondly remembered than the others here, thanks to acting as Applegate's big final story before turning the series over to ghostwriters, plus Jake and Cassie's Relationship Upgrade.
For a non-Sario Rip example, there's The Prophecy. A sequel to The Hork-Bajir Chronicles where a recording of Aldrea's personality is put into Cassie, which would be fine, except the process comes off as pure voodoo that sticks out strongly in this mostly hard sci-fi series, and Aldrea is inexplicably considered to have chosen to become a Hork-Bajir nothlit rather than being knocked out. Plus, there's a glaring editing mistake that makes the Animorphs look like idiots when they're all shocked and appalled that Aldrea doesn't known where the book's MacGuffin is, after she already told them.
The Underground, which shows the Yeerks to have an addiction to maple and ginger oatmeal, which drives them crazy with extended consumption. Near the end, Jake comes right out and says that this whole plot is stupid and this battle will be forgotten by history.
Jake: You know, from the point where Edelman said "maple and ginger oatmeal," I should have known this was going to end stupidly.
Cassie: Instant maple and ginger oatmeal.
Jake: Battles that involve oatmeal are just never going to end up being historic, you know? Gettysburg? No major oatmeal involvement. The Battle of Midway? Neither side used oatmeal. Desert Storm? No oatmeal.
The Mutation, in which the Animorphs find an Expy of Atlantis, then escape, and it's never brought up again, with the whole thing being an excuse for an Anvilicious "War is bad" message. It also doesn't help that the book's ghostwriter was a comedy writer way out of her depth.
The Separation, in which Rachel gets split in half while in starfish morph and the halves regenerate into two Rachels that are both Flanderizations of certain character traits of hers.