In the Conan the Barbarian (1982) movie with Arnold Schwarzenegger, when Conan tells Thulsa Doom about how he slaughtered Conan's village and had him sold into slavery, Thulsa Doom does not remember at all. Though subverted in that he does at least remember the why of how it happened when Conan presses him.
Conan: You killed my mother, you killed my father, you killed my people! You TOOK MY FATHER'S SWORD!! Thulsa Doom: Ah. It must have been when I was younger. There was a time, boy, when I searched for steel, and steel meant more to me than gold or jewels.
Batman:You killed my parents. Joker: Wait...[chuckles]...what are you talking about? Batman:I made you. But you made me first. Joker: I was a kid note according to the script, Jack Napier was in his early twenties at the youngest - not a "kid" and definitely old enough to know better when I killed your parents. When I say "you made me", you gotta say "I made you"! How childish can you get?
This hasn't stopped some from theorizing that maybe Batman was wrong, however.
Fools Of Fortune: The protagonist finally meets the man who murdered his family and burns down his village. He doesn't even remember the event, causing a major Berserk Button for the main character.
When the title character accuses the Dread Pirate Roberts of killing her love, Westley, he replies "It's possible. I kill a lot of people." Then subverted, because after she actually tells him something about her love he says he vaguely remembers someone who might have been him, someone who didn't beg for his life but just asked to be spared, because of a woman he loved. He actually knows exactly who she's talking about because he is Westley.
When Inigo finally confronts the six-fingered man he's been hunting his whole life. Rugen does vaguely recall him, however.
"You must be that little Spanish brat I taught a lesson to all those years ago. You've been chasing me your whole life only to fail now? I think that's about the worst thing I've ever heard. How marvelous."
There's a comedic example, with various women slapping Jack across the cheek, to which he blithely replies either "...I'm not sure I deserved that" or "...I probably deserved that one." The one time he actually remembers why a woman slapped him was because he "borrowed" her ship. He remembers ships but not women.
Earlier in the movie it's subverted when Jack says to Will "You look familiar. Have I threatened you before?" He hadn't, but rather was seeing a resemblance to Will's father whom he knew well.
In Mobsters, one of the driving motivations for "Lucky" Luciano wanting to kill Don Faranzano was the death of Lucky's father years before, something the aged gangster had forgotten as a mundane event.
Don Salvatore Faranzano: At least tell me what I did 15 years ago.
Lucky: You destroyed my father!
Don Salvatore Faranzano: I don't even *remember* your father!
Once Upon a Time in the West: Ruthless killer Frank is being pursued by a mysterious drifter known only as "Harmonica," but Frank has no idea why, nor can he remember who Harmonica is. Harmonica never reveals his own name; whenever asked, he instead gives names of some of the many people Frank has killed over the years. His final clue is sticking his harmonica in Frank's mouth, which factored into the event that Harmonica wants to avenge.
Oldboy (2003). Oh Dae-su has long forgotten that he witnessed Lee Woo-jin's incestuous relationship with his sister. Unusually, Woo-jin has no illusions about the scale of the incident from Dae-su's point of view, and doesn't actually expect him to remember it.
Freeway Killer begins with William Bonin talking with the mother of one of his victims. When the mother shows him a picture of her son, Bonin nonchalantly says "So many faces, they all just get so...mixed up".
In Pat Garrett And Billy The Kid, at one point a group of gunman try to take on Billy's gang, led by one in particular who keeps insinuating that he knows Billy, but won't reveal how he knows The Kid. The ensuing shootout is a Curb-Stomp Battle in favor of Billy's gang, and afterward Billy wonders who the men were, concludes that it's far too soon since his latest escape from prison for bounty hunters to be after him, and says, "I guess it really must have been something personal." He thinks about it for another few seconds before he gives up trying to figure out who the guy was and goes back to eating lunch.
The titular hero of Forrest Gump doesn't seem to notice when people laugh and refuse to believe the events of his life because he never recognized how epic and momentous many of them were. This guy inspired Elvis, saw his college get desegregated at gunpoint, was a Vietnam War hero and peace activist (albeit accidentally), tipped people off about Watergate, was an exercise guru and provided seed money for Apple computers...after all that, creating the smiley face really was just a regular day for him.
In The Quick and the Dead, Ellen's entire motivation in entering the Quick Draw competition is to kill Herod, but Herod has no idea who she is. Justified in that he last saw her as a child. When she produces her father's badge, he recognizes it instantly.
Both parodied and subverted in Kung Pow! Enter the Fist when Betty initially doesn't recognize the Chosen One, but does upon seeing his baby booties.
"Sorry. I didn't recognize you without crap in your pants."
The Damned United: Revie is honestly taken aback that Clough has been harboring such a grudge for so long, over an incident he himself didn't even notice. The real-life facts are more ambiguous, but the film adaptation plays the trope straight.
The Big Bad in The Losers doesn't remember setting up the titular team at the beginning of the movie when reminded of it later, as he does this a lot.
Double-subverted in Last Action Hero: the Trapped in TV Land protagonist warns Cowboy Cop Jack Slater (played by Arnold Schwarzenegger) not to trust his Big Bad Friend John Practice (played by F. Murray Abraham) because "he killed Mozart," a reference to Abraham's role in Amadeus. When Practice does eventually betray Slater, Slater repeats the accusation to him, mangling it as "you killed Moe Zart." Practice's response? "Hey, I kill a lot of people. I can't remember half of them."
Subverted in The Decoy Bride. Lara yells at the paparazzi photographer who's hell-bent on getting photographs of her wedding that he's ruining her life, and he answers that "I've ruined plenty of lives. You're nothing special." Later it turns out that he was lying; she is special to him, and he puts a lot more energy into following her around than he does any other celebrity because he's in love with her.
The events of "Space Seed" were this for Kirk in Star Trek II: The Wrath of Khan. As director Nicholas Meyer puts it, "I think when Khan makes his appearance in the story, Kirk is flabbergasted. He did not lie awake thinking about Khan. Khan lay awake thinking about Kirk."
Understandable in that as Kirk saw it, his dispute with Khan had been settled and as far as he knew it had worked out well for both sides. He certainly had no idea that the planet he left Khan on would soon turn into a hellhole leading to the deaths of most of Khan's followers and his wife.
Happens to James Bond in The Spy Who Loved Me, when Soviet spy Anya Amasova questions him about her lover, who Bond had killed during his mission in the opening teaser. (However, to his credit, he does at least still remember killing someone.)
Bond: When someone's behind you on skis at 40 miles per hour trying to put a bullet in your back, you don't always have time to remember a face. In our business, Anya, people get killed. We both know that. So did he. It was either him or me. The answer to the question is yes. I did kill him.
In Dredd, the titular Judge goes to investigate a few grisly murders and finds himself sealed inside a massive 200-story apartment complex ruled by a sadistic drug gang that wants him dead at any cost. At the end, Judge Dredd dismisses it as a typical day for him.
In the Soviet adventure / musical comedy Fear Not, I'm With You, a bandit leader orders his henchmen to torture an old peasant (for information he doesn't even know!).
Old man: Beware! This sin Allah won't forgive you!
Bandit: If you only knew how many sins there are on my soul... One more... one less... Neither me, nor Allah would notice.
Played with in City of God. After Lil Ze accidentally kills Benny while they were arguing, Lil Ze blames Rocket for his death, but doesn't do anything to him. Months later, when a photograph Rocket took got leaked by the news studio he works for, Rocket believes Lil Ze will remember him and have him killed. Instead, Lil Ze forces him to take photos of his whole crew, all while seemingly not remembering who Rocket is, as he often asked his men the name of the photographer. After the last day of take pictures, he lets Rocket keep the camera and spared his life, telling him that's what Benny would've wanted. Benny was friends with Rocket and gave him the camera as a gift.
A protagonist example in The Amazing Spider-Man 2. Spider-Man saves Max Dillion's life while he's busy chasing down some Russian mafia goons stealing plutonium. Spider-Man gives him a few words of encouragement and moves on, unaware that Max is a bit on the obsessive side when it comes to anyone acknowledging him. After Max has transformed into Electro and is met by Spider-Man once more, he gets irritated when Spider-Man doesn't remember the incident. After a reminder, Spider-Man does in fact recall that day, but can't remember Max's name unprompted. Spider-Man almost talked him down regardless, but the police messed it up by shooting at Max and driving him into Unstoppable Rage.
To be fair, though, Max's physical transformation was so severe it's not surprising Spidey didn't recognize him or connect him with the original event.
A protagonist example in the UK crime thriller Blitz. The antagonist, Berry (played by Littlefinger), started killing cops, after he got severally beaten up at a night club by Brent, a quick tempered police detective played by Jason Statham. During the interrogation scene later in the film, Brent makes it clear that he forgot all about that night at the club, until he saw Berry's face again, and even then, wasn't sure until he linked him to the cop killings. He also admitted that his fellow officers got a good laugh watching the security video of him getting beat up back then.
Drax of Guardians of the Galaxy lost his family to antagonist Ronan the Accuser. When the two finally meet, a Curb-Stomp Battle ensues wherein Ronan states that he has no memory of killing Drax's family... and that he will probably not remember killing Drax, either. In a later confrontation Ronan claims to have since recalled killing Drax's family (and cruelly remarks how he remembers how they "screamed pathetically"), though this could very well have been an effort to enrage Drax further. Tellingly, he mentions no other details.
Taken 2 has a rare heroic example. Krasniqi shows Bryan photos of some of the men he killed in Paris and is disgusted that the only man Bryan recognizes is Krasniqi's son, whom Bryan remembers as the kidnapper he spoke to on the phone and later tortured.
In Magnolia, Jimmy Gator can't actually remember whether he sexually abused his daughter. If so, it is likely the event that sent her down a drug-addled spiral.
Subverted in Rock of Ages: Stacee Jaxx really does remember the night long ago when he broke the heart of Mayor Whitmore's wife, who was an adolescent girl at the time and subsequently became a crusader against Heavy Metal music, although he doesn't show more than token remorse about it. And Patricia Whitmore is in the end able to let all of that go and rejoin the subculture she had despised for so long.
In the Dirty Harry franchise film Sudden Impact the woman protagonist goes on a Roaring Rampage of Revenge to kill the men and a woman who gang raped her and her sister. Many of them she confronts don't remember her or even what they did.
The titualr character of Hancock apparently doesn't remember an incident where he sank a yacht while throwing a beached whale back into the sea, even after seeing footage of it on Youtube.
Iron Man 3 has a heroic example with Tony Stark. Thirteen years ago, he slept with Maya Hansen one night and helped give her the correct equations to develop the Extremis formula. When they meet again in the present, she's insulted to learn that he has no memory of these events.