Jonas's father killing a baby, and cheerfully saying "Bye bye, little guy!" as the body goes down a metal chute.
This scene is even worse if the reader is a twin... especially a lighter one. "It could've been me!"
And in the movie, it's worse! Watching the baby's body go VERY limp after the injection is just shudder inducing. It's worse than a similar scene in Trainspotting.
This is even scarier when you remember that while to us the act of putting a syringe in someone and they stop moving is horrific, to the people doing it, it's just "release to elsewhere" and "good luck to them; time to do something else". A terrifying act made even more terrifying by the fact that the people are taught it's as nice as a walk in the park.
It's worth noting that in the movie, Jonas's father DOES eventually realize what he's really been doing (when Jonas's memories return to the community)- and he is utterly horrified.
Gabe slated to be released because he couldn't sleep through an entire night without Jonas there to give him soothing memories.
Some of the memories Jonas receives qualify as this. Remember the first "bad" one? "Oh look, it's the sled again! This can't go wrong-..oh...OH....aaaagh..." And remember, that was somebody's memory, meaning that horrible crash and mangling happened to someone.
To top it off, Jonas isn't allowed to use painkillers to deal with the painful memories. As if torturing him with the pain of somebody breaking his leg wasn't bad enough, now he has to go through the rest of his day with not only a bad memory on his mind but still feel the aftermath...and isn't allowed to take something to dull the pain. Fridge Horror sets in when you realize that the no painkillers rule may also extend to things like antidepressants and antipsychotics.
Even worse when you remember that The Giver (and every Giver/Receiver before him) has to deal with this all the time. It's enough to make you wonder how the previous Givers and Receivers even stayed sane.
How about that poor sap of a Pilot at the beginning? A mild navigational error led him to fly over the town as he tried to get back on course, and on schedule. Because of this, the Elders actually considered shooting him down; the Giver talked them out of that, but he was still Released because of the impact his actions had on the community, never mind that it was a mistake.
Not to mention, since he's a pilot-in-training, he's probably around 14.
Also what the community members went through during the experience. For them, this is literally something so out of the ordinary as to be incomprehensible. They can't even start thinking in worst-case scenarios like people today would; it's so completely outside their frame of reference that they can't begin to imagine what might happen next. It's resolved quickly, but that had to be a terrifying few minutes.
On a related note, Asher, of all characters, apparently went silent for a while (and was left with marks on his legs) after he was physically punished for repeatedly getting his words mixed up (he would ask for a "smack" instead of a "snack"). It can make the readers wonder how many other kids this has happened to, and what their reactions would be if they were already fairly quiet in the first place.
The recollection by the Elder is also chilling because it's treated like a funny anecdote, like a parent telling a story about an embarrassing thing their kid did that's funny now.
On that note, Fiona notes to Jonas that the elders are also punished whenever they make mistakes. Remember that there may still be conditions that cause senility in adults in the Community so it's not necessarily their fault when something happens.
The breadth of the Giver's range of memories - memories from all over the world, and dating back into and before our present time - can become a little creepy if one considers that, if the Community is supposed to be our own future, he's got memories ofthe reader locked away in there, too. Including each and every memory you'd prefer not be psychically handed off to generations of total strangers.
Birthmothers receive their Assignment at Twelve, like everyone else. About a year later, they are sent to the Birthing Center. They are impregnated - a process they no doubt do not understand at all - starting around fourteen. If, like Claire, something goes wrong, they are summarily dismissed. Even if everything goes right. three births, roughly five years, since they get some time to recover, so around age 17, they are then re-assigned as Laborers for the rest of their lives.
The replacement child Caleb - To a kid, replacing a child who has died with a new one is innocent enough, even kind of nice with the accompanying Murmur-of-Replacement Ceremony. But to most adults, and especially parents, who realize the loss of a child is an unimaginable tragedy and children cannot be replaced, the ceremony is unnerving.