Forgotten First Meeting

Many years ago, Alice and Bob met each other by random chance, and the meeting left a deep impression on at least one of them. But years have passed, and now they're grown up (or at least in High School). All unknowing, they have been pulled back together by the Red String of Fate - friendship, romance and/or adventure will see them rekindle a long-lost connection long before they realize that they've met before. The realization that their new friend is the same kid they met years ago may come slowly with heavy foreshadowing, or be a sudden twist.

A classic setup for the romantic anime, when the Childhood Marriage Promise would be a bit too blatant. Quite popular in western shows as well, however.

Related to Already Met Everyone and Childhood Friends. Contrast Forgotten Friend, New Foe, Old Flame Fizzle.

Anime and Manga

 * Nogizaka Haruka no Himitsu: It eventually turns out that Yuuto was the one who got Haruka into anime and manga in the first place when, as a kid, he found her crying in a playground on his way home from a shopping-trip with an Otaku friend, and gave her a manga magazine. The incident didn't leave a huge impression on him, but it essentially changed her life - arguably for the better. (Though her Overprotective Dad might disagree...)
 * In Ai Yori Aoshi there's a broken Childhood Marriage Promise but she wants to find him. When she does he doesn't remember her at first. There are flashbacks, not to the first time they met but while the were friends as kids together.
 * Komari and Yukio in Gokujou Drops.
 * Pokémon: Barry doesn't remember that he and Dawn met when they were younger. Granted, they're probably only ten or eleven...
 * In Mamoru Kun Ni Megami Ni Shukufuku O, it gradually becomes clear that the Beatrice-user who saved Mamoru during a disaster when he was small, was Ayako's uncle - and what's more, she was there too. Neither one of them remembers at first due to having been quite young and traumatized to boot, but that event was the reason why Mamoru decided to become a Beatrice-user himself, which led to him joining the Beatrice Academy, where he, of course, met Ayako again...
 * Love Hina is probably the poster child for this trope.
 * Fruits Basket's Tohru met Yuki when they were younger, but it isn't until later on in the series he realises she was the same girl. He believes she doesn't remember but actually the memory was very important to her, even though she never learns that Yuki was the boy.
 * In Hyakujitsu no Bara, Klaus and Taki meet briefly in the middle of a forest by chance when they are young and make something of a Promise. Years later, they meet again and Klaus recalls but continually dismisses the memory, until their Relationship Upgrade, when he realizes from Taki's scent that it was him.
 * In Tokyo Babylon, Subaru meets Seisshirou as he cradles his kill, and they end up making a promise. Years later Subaru only remembers this as a vague dream, and has little idea why Seisshirou has suddenly appeared in his life as a suitor. Unfortunately the memory isn't a very happy one, once it's revealed.
 * In the Saint Beast drama cds, Rey reveals to Shin that he had met Luca long before the day he actually introduced himself, and felt guilty for hiding it. In their adolesence Luca found Rey crying and lost in a forest and led him out. After searching out his rescuer, Rey was too embarassed to introduce himself until many years later when he received wings and felt he had something to bond over. Shin remarks that Luca probably knew all along and was too gracious to say anything.
 * In Junjou Romantica, Nowaki recalls the day he found out the orphanage owners weren't his real parents and tried running away. He stops at a park when his tires go flat, only to have his future lover walk by, inquire, and then start dishing out life lessons about the obstacles kids must get through before tattling to the patrolman. Years later Nowaki remembers the encounter and mentions it to Hiroki, but neither realizes it was each other.
 * In Spirited Away, Mysterious Protector Haku knew Chihiro's name even though he couldn't remember his. Chihiro kept having a feeling that she met Haku from before but couldn't remember when.
 * Futari wa Precure Splash Star : Both Cures met briefly five years prior to the events of the series and forgot.
 * In Axis Powers Hetalia, Germany is hinted to be Holy Roman Empire, Italy's Love Interest when they were little. It's not clear whether Italy believes them to be the same person or not, but Germany, at least, appears to have forgotten about their shared childhood. He might have remembered in the Valentine's Day strip, but that plotline has been abandoned for years now.
 * In Skip Beat!, Ren and Kyouko actually met when they were young children, not when they were first introduce at the agency.  Ren knows, but Kyouko is still in the dark about their pasts.

Comic Books

 * In The Phantom the hero and his love interest meet for the first time in the first story arc, until Lee Falk wrote the story of "The Childhood of the Phantom" and included a scene where they met briefly years before when they were both children.

Film

 * In The Princess and the Frog, Tiana and Naveen meet minutes after he arrives in New Orleans. He tries to woo her, but she ignores him and he moves on, even saying "Eh..". He fails to recognize her when they meet again at the party, though.
 * Brian Allen and Christina Roberts in Blow Dry. They knew each other as very young children because their fathers were business partners. But then the partnership broke up, Christina's parents divorced (and she went to live with her mother in Michigan), Brian's parents divorced and he moved in with his dad in York, and the two lost touch with each other for twenty years. Cue the events of the film...
 * The prologue to the movie Adam and Steve features the two main characters hooking up in the 80s and having spectacularly bad drugged-out sex. They meet, sober and professional, 20 years later and don't realize it's the same guys till almost the end of the film.
 * This happened in the new Nightmare On Elm Street remake to the protagonist.

Literature

 * In King Kelson's Bride, Kelson is reminded (by Rothana) that he had not only met his cousin Araxie when they were children, he also played with her and they got along very well with each other. Though their marriage is arranged, Kelson and Araxie decide to use those happy memories to build upon when forging their relationship.

Live Action Television

 * In Fringe Peter met Olivia as a kid, but seems to have amnesia about this ever happening even though he is  now.
 * They both forgot this meeting as a result of repressing traumatic memories from around this time: for Peter, ; for Olivia,.
 * In Community Jeff and Shirley realize that when they were children Shirley humiliated him in a game of foosball. Jeff moved away soon after and they did not meet again until they were adults. The humiliation turned Jeff into the Jerk with a Heart of Gold he is today and Shirley's Heel Realization made her a very passive person.
 * The Odd Couple: in one of the several versions of how Felix & Oscar first met, it was when Oscar's father ran a speakeasy in 1920s Chicago and Felix's father was an optomitrist who fitted Oscar's father with glasses.
 * My Name Is Earl: Randy goes to meet his childhood crush "Pinky," who it turns out was Earl's ex-wife Joy.
 * In one episode Drake and Josh discover that they met when they were nine (rather than in high school as they previously thought).
 * In Iljimae, one of his love interests figures out who he was by a piece of jewelry the boy who saved her life carried. The other love interest also figures out who he was because he continued a story he never finished when they were children.
 * Dharma and Greg first saw each other as children when they barely missed meeting each other on a subway. Twenty years later they met on the same subway and fell in love.
 * An episode of Mad About You reveals that Paul and Jamie first met as kids at the Natural History Museum.
 * In "Friends" Ross finds out that Phoebe robbed him when they were kids.

Theater

 * Gilbert and Sullivan gives us Patience - Patience and Archibald were best friends when they were children; they are adults and meet again. Romance Ensues.

Video Games

 * The second Blaz Blue reveals that as a student Ragna met Taokaka while she was still a kitten (although he might remember, she doesn't). In this case their relationship is strictly platonic, however.
 * A sequence in Final Fantasy VIII shows that either all or virtually all of the party members spent time in the same orphanage as children, and most of them don't remembered it - much less the fact that the person running said orphanage was . Irvine is the only one who remembers it all, due to not having used Guardian Forced until he joins the party - as it turns out, they can cause memory-loss with sustained use.
 * Persona 2: Innocent Sin groups together Tatsuya, Maya, Lisa and Eikichi. They were all childhood friends of a group called the Masked Circle  Note that all the Love Interests for Tatsuya are from this group.
 * In Fire Emblem Tellius, Ike and Soren had met years ago, and Ike did not remember the first time that he met Soren, although Soren remembered in vivid detail. It was not until near the end of Path of Radiance that Soren bothered to remind Ike how they met, and it wasn't until Radiant Dawn that Ike finally remembered it first-hand. Justified in that their first meeting was right before Ike saw, for which   which wasn't cured until being in the tower, where   memories come to the fore.

Visual Novels

 * This trope plays a large role in the 18+ Otome Game Under the Moon, in which it is paired with Laser-Guided Amnesia and Played for Drama.

Web Comics

 * Too Much Information: The latest addition to Ace's Unwanted Harem is a very tall Ainu girl who ran away from home because she was always being mocked for her size, determined to find the only boy who'd ever called her pretty - a young Gaijin boy she'd found lost back in Hokkaido, and helped to find a police-box. Cue the revelation that Ace had gotten lost during a trip to Hokkaido with his mom when he was a kid, and had been helped by a pretty redhead...
 * Kevin and Kell does this with Rudy and Fiona, who find out after dating for some time that they were in the same preschool - and hated each other.

Western Animation

 * The Simpsons: Marge & Homer met when they were circa 10 years old at summer camp and shared their first kisses together. When they met again in high school neither recognized the other (in part because one had very different hair and the other had an eye-patch when they first met).