It is a shock, when Bunny mentions the Blizzard of 68, just fooling around, the pair of them exchanging barbs that after a decade of camaraderie don't sting anymore, and Jack looks at him quizzically and tells him that there's never been a blizzard in 68 – certainly not on Easter Sunday. He wouldn't do that. Bunny laughs it off with a simple "I know you wouldn't Frostbite" but he can see that Jack is still confused. It's a shock to hear Jack deny the Blizzard. It's unsettling to realize he means it.
Pitch has been silent for nearly a century, but that doesn't mean that North is letting his guard down. So it's a surprise when Jack does. When, after another practice drill in the event of an attack on the pole, Jack lands lightly on the back of his chair and asks North why they bother with the drills. It's not like anyone could ever seriously face the Guardians and win. North is just ready to ask incredulously about Pitch's attempt when he sees a flicker go out on the globe. It's Burgess, Jamie's great granddaughter, age 10. They both feel the removal but Jack doesn't react other than to lament that children must grow up. North knows that Jack can sense which individual child had stopped believing and so it's a surprise to North, even though he knows that Jack stopped keeping tabs on the Bennets after Jamie died in his early thirties, a believer till the end, that Jack doesn't have more to say on the loss of this light. It's frightening when North asks about Jamie and Jack asks, "which one?"
Tooth is aware of Jack's memory lapses. Memory is her specialty after all and everyone is worried about their youngest member. But it doesn't really hit home until the day Jack flies into her palace asking after his memories. That isn't the blow: though he hadn't watched them again after the battle with Pitch, she'd only expected him to want to look at them again eventually. The blow is what comes next, when Jack asks her, "I know you have the memories of everyone else, I was wondering if mine were in here somewhere?"
The day Sandy realizes that Jack doesn't really remember a time without the Guardians at his side is the day they understand how he survived three-hundred years of isolation and ridicule without cracking. It's not that it doesn't hurt. It's not that he doesn't somehow know that something's missing where his past should be. It's that it doesn't matter. Somehow Jack will always be just a child, with a child's dreams, a child's hopes, a child's wonder and a child's fears. But none of that can come without this last. Their Jack will always have a child's memories. He'll never have to remember the march of centuries. Someday he'll forget even the shadow of abandonment in favor of the love of his family. He will ever retain the innocence of the children he protects.
After all, what is memory worth when compared to the world of a child who will never, ever, grow up?