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."
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."
An unusual inversion in Star Trek II: The Wrath of Khan: Khan stayed up every night for twenty years dreaming of revenge against Captain James T. Kirk; Kirk likely hadn't thought of Khan at all after he filed his report to Starfleet on the Space Seed incident. The fact that Kirk and Starfleet never checked up on the new colony, essentially abandoning them to die when a natural disaster hit, is a big part of why he's out for revenge.
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.
A rare heroic example in The Amazing Spider-Man 2; Spider-Man helps so many people on a daily basis that he doesn't remember Max Dillon until Max reminds him of that day. Technically speaking, Spider-Man does in fact remember Max, just not his name, and Max's transformation into Electro hasn't exactly made him easier to recognize.
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."
Iron Man 3: The origin story for Aldrich Killian features this, with a joke being played on him by Tony Stark being the catalyst for his villain origin story.
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 their "screams were pitiful"), though this could very well have been an effort to enrage Drax further. Tellingly, he mentions no other details.
This is discussed by Zemo in Captain America: Civil War. After the climax of Avengers: Age of Ultron, the Avengers, satisfied that their work was done and their latest adventure was over, went home to America. Meanwhile, Zemo was left in the debris and rubble with his now dead father, wife, and son.
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.
Rampage: In the second film Bill is confronted by a young woman who says he killed her twin sister. He remarks that she will have to be bit more specific, as he's killed too many to remember their faces. Turns out it was the waitress at the fast food restaurant from the previous film, whom Bill killed for merely spilling some food on him.
Dredd: For Anderson, the events of the film are a hellish baptism of fire that completely shakes her to the core and changes the way she sees herself and the world around her. For Judge Dredd, her supervising officer, it was just another drug bust.
Nine Dead: The entire mystery hinges on Sully, a mob-connected Loan Shark, having once loaned five grand to a rookie criminal, who then robbed a convenience store to pay him back and let an innocent man take the blame. When he finally learns this, he's astounded that of out of all the bad things he's done in his life (and he's done far worse), some small deal that he never paid any mind to again is the thing that comes back to haunt him.
John Wick: Chapter 2: The Bowery King says back when he was a low level mook, he got into a fight with John Wick that ended with him stabbed in the throat. John spared his life, and after he recovered, he reinvented himself and rose high in the criminal underworld. The Bowery King considered that the defining moment of his life, but acknowledges John wouldn't remember it as it was just another fight to him.