The American Library Association’s Office for Intellectual Freedom keeps track of the reasons why books are challenged. One of the most common reasons is "unsuited to age group." Honestly, though, there is an awful lot of stuff out there that adults find inappropriate that goes right over kids' heads.
To use a non-literary example, I recall a slumber party my sister had where my mom let us watch Mrs. Doubtfire. I was 10 or 11, and my sister and her friends were four years younger. Re-watching the movie as an adult many years later, I said, "Oh, my god, I can't believe my mom let us watch this!" But when we were kids, we didn't understand the racy jokes - when we were kids it was just Robin Williams in a dress doing goofy things like dancing with a vacuum cleaner.
A few years ago, there was a big to do in the county where my parents lived about a book that was on the summer reading list - Z for Zachariah. A lot of parents were upset because a teenage girl in the book is assaulted by an adult man. My mom asked me if the book was "really that bad," because she remembered that I had read it in middle school. Honestly, I didn't remember any kind of assault. I remembered that the girl was trying to get away from the man, and that I had been far more upset by the dog dying.
Similarly, there was another book I read around the same rough time/age, Julie of the Wolves. Unlike Z for Zachariah, this one is on the national banned books list.
In the book, the main character, Julie, is left with her father's friends when her father goes missing. At 13, they arrange for her to marry their son, who is a couple years older and a little slow. She is told they will be "like brother and sister," and so she goes along with it. Daniel, her new husband, barely speaks to her, let alone anything else. But one day he comes home and says that his friends are making fun of him - "Dumb Daniel - he's got a wife and he can't mate her." Julie points out that "we don't have to." Daniel counters that "they're laughin'." He tries to kiss Julie; she pulls away. He trips her and they fall to the floor together. He sticks his tongue in her mouth and she kicks him. He lays on top of her for a moment but tries nothing else. Then he gets up and runs out of the house proclaiming, "Tomorrow! I can!" Julie rolls over and throws up, then immediately begins packing her things.
Reading this as a 5th grader (I think), I had a general idea of what "mating" meant, but I saw this scene just as Daniel forcing a kiss on Julie an generally being mean. Apparently a lot of parents read more into it. My mom asked me about this one when I was a children's librarian too, asking me if that scene had bothered me (she had read it after I did) and lamenting that she had let me read it. Since this one also shows up frequently on the banned book list for being “unsuited to age group,” I decided to reread it with an adult mindset; it seemed I had obviously missed something.
But, no, on rereading it, the scene I describe above plays out pretty much as I have written it - no fade to black, no "half an hour later" or "the next day" or "after what happened." I mean, it’s possible to read that scene and find somewhere that more can be inserted than what's already on the page (that’s always possible). However, Daniel getting up and proclaiming “tomorrow!” kind of clinches it for me that he didn’t accomplish what he set out to do, especially since it’s pretty clear that Daniel has no idea what he's really trying to do.
Rereading the book as an adult, I also caught other things, wonderful details that I completely missed as a kid. Julie and her father are Inuit. Shortly before Julie's father goes missing, he tells her that "the government is fighting a war" and they want him to go fight, too. He takes his kayak out hunting and doesn't return. Later his kayak is found smashed on the shore and everyone assumes he has died. At the end of the book, though, Julie discovers that he is alive. As a child, I hadn't even remembered the brief mention of the war and the smashed kayak; as an adult, I had a strong suspicion of what was going on. I flipped to the front of the book. Seeing the publication date of 1972, it confirmed my suspicion that the war was Vietnam and Julie's farther faked his death to get out of being drafted.
There were other things I noticed on the reread - color description, metaphors I had taken literally, and literal passages I had taken metaphorically, and generally just really good writing that I hadn't appreciated when I was a kid. I love that, though - that a book that is written for kids has multiple layers so that it can be reread and appreciated over and over again.