This may have been mentioned already but here's an obvious example of leading the audience one direction and then flip-flopping when it suits the plot, and a bonus plot device that contradicts itself:
This is the sort of teen-aimed sci fi book that really is juvenile. At this point in the story, the Heroine and her Sidekick are shipwrecked on an alien planet. When Heroine first meets the locals, she is panic-stricken at their hideous, beastly appearance (picture a race of particularly feral-looking Wookiees).
The locals dispel this impression within hours by being nice to her, curing her injuries, and living in a wonderful forest paradise. Also they speak (stilted) English because of course they do.
After a few more hours, Sidekick discovers that the locals really are war-mongering sociopaths who only haven't killed them because they might be useful, but will gladly rip them to shreds when they are no longer useful. When he tells Heroine, she is shocked - shocked, I tell you! - to learn this about the hulking beasts with huge fangs and claws that scared her half to death when she first met them... 5 or so hours earlier. But- but- but- she says, they didn't immediately kill us and they live in such a beautiful place! Yes, the beauty of their land is actually used by Heroine as a reason why she suddenly can't believe they could be so vicious to each other.
Also, the locals are technological geniuses who can remotely control a starship but CHOOSE to live with no technology because it's against nature, and so they destroy all the tech they find using their unbelievably advanced tech skills. (How they control the previously unknown to them starship remotely from a planet's surface, much less knew it was there in the first place, without actual technology is glossed right over.)