The Dune Encyclopedia names House Washington (AKA the US) as the first user of atomics in a "provincial war."
In Interesting Times, near the end of the book, Twoflower is quite aware that Lord Hong has no idea he was the cause of his wife's death, but he insists on fighting him anyway. Twoflower says that the fact that Hong didn't know makes it worse.
Redwall's Lord Brocktree: Bucko Bigbones confronts Karangool, the fox who killed his family and left him for dead, and Karangool has no idea who he is. Bucko corners him and explains before taking his revenge. And then he does remember.
"Well, let me tell ye a story, aboot a puir young hare who was left fer dead by a wicked ole fox who beat 'im wi' a swordblade..."
Usually almost all villains never bother to remember who their slaves/prisoners/murder victims are are unless they either "Stand out by how strong they are" or "The slave/prisoner escapes, and the villain leader immediately goes berserk to get said slave/prisoner back" (not because s/he is personally worth anything to the villain, but simply because s/he defied the leader)
In the second book of The Baroque Cycle, Jack Shaftoe confronts the man who sold Eliza into slavery before killing him, tells him that he is taking revenge for a woman who he enslaved. Since the guy has done this countless times, he is genuinely puzzled for the last few seconds of his life.
In Life, the Universe and Everything, Agrajag is convinced that Arthur Dent has this attitude towards him, since Arthur is always the one who kills him in his various reincarnations; he even has a citadel dedicated to his hatred for Arthur, centered on a statue depicting him as a multi-limbed ogre killing innocent creatures (all former lives of Agrajag) carelessly. For his part, Arthur just thinks the universe is playing silly buggers with them. Which it probably is.
Vaun: Tell me what I have done to earn such enmity. Verity: You...You don't even know? Does killing mean so little to you that you dismiss it from your mind with every murder? Vaun: For the most part, yes. Let me see if I can guess. A father? Or a brother, perhaps?
He does remember eventually, after some jogging of his memory.
Verity: My sister, Lethe Catena [...] You ended her like some common animal! Vaun: Ah. Of course. There's a bit of family resemblance between you, isn't there?
Older Than Television: In the short story "The Procurator of Judea" (published in 1902) by Anatole France, the last line is spoken by a retired Pontius Pilate:
"Jesus?" he murmured, "Jesus — of Nazareth? I cannot call him to mind.".
And then Zig-Zagged when it turns out that in the villain's obsession with the hero he himself has completely forgotten about Lily AKA Erica
In the Stephen King short story "Dolan's Cadillac", the wife of the protagonist/narrator (Robinson) is killed by a crime lord she was going to testify against (Dolan). For a few years or so, Robinson follows Dolan to learn his habits and routine, all while plotting his revenge. During one harrowing incident, however, Dolan's car breaks down on the road, and Robinson is forced to pass him. He's angered when Dolan doesn't even recognize the man who's wife he had ordered blown to smithereens in her car. This is then subverted when, after Robinson says the first few words of his Best Served Cold speech, Dolan immediately identifies him.
The original title of All Quiet on the Western Front is Im Westen nichts Neues ("Nothing New in the West"). It ends with the last surviving character getting shot by an unseen sniper followed by a Title Drop. For him it was the day he was killed; for the war, it was just an ordinary day with nothing to report.
In the Iain M. Banks novel Inversions, the Lady Perrund becomes courtesan to the King after a rival warlord's soldiers killed her family and raped her. It is only at the end of the book after she has murdered the King that we discover it was actually him and his men that committed the atrocity. The King had never recognised her and had most likely forgotten the incident, allowing her to get her revenge.
Non-villainous example in Tamora Pierce's Protector of the Small Quartet. Kel has a fear of heights due to her brother dangling her over the edge of a tall tower when they were kids. When speaking about it, she says that the worst part was when she spoke to him about it: he didn't remember.
The protagonist of Howl's Moving Castle assumes this trope will apply to the witch who cursed her, and therefore that she'll be able to pretend she has no connection to the witch. Unfortunately, she underestimated how much the witch hated her.
Realm of the Elderlings takes place in a fantasy kingdom composed of six duchies- four coastal, two inland. The royal family is based in a coastal duchy. In one throwaway conversation in the first book, Chade grumbles to Fitz that the people blame the king for everything from "bad weather to fires". In the third book, Fitz travels inland and discovers that, years ago, huge swaths of vital farmland were ravaged by fire and have never recovered, causing widespread hardship and economic depression. Fitz then realises just how out of touch the coast and inland are with each other- for the coast the fire was a minor annoyance, but to the people living there, "it must have seemed like the end of the world"
Star Wars: Shadows of the Empire: Prince Xizor is attacked by a young man named Hoff, who yells "You Killed My Father" to him, to which Xizor replies, "I'm sorry. Have we met?". It's averted, however, since Xizor remembers the father once Hoff tells him the father's name.
Amusingly, Xizor hates Vader because he blames him for the death of his family. Vader is utterly oblivious until one of his spies informs him. Vader likewise remembers when told about it, he simply had no idea that Xizor was related to any of the people he killed that day, and thought Xizor merely coveted his place as the Emperor's right hand man rather than being somebody out for vengeance. Xizor did seek to usurp Vader, but that would've simply been part of the revenge.
In one short story a bounty hunter named Gurion Silizzar corners Dr. Evazan and asks him if his family name sounds familiar. The doctor says he may have had a patient or two, to which Silizzar angrily tells him that seven of his family were killed in his experiments.
In The Kite Runner, Assef, the main antagonist, recounts the abuse he took at the hands of the Soviet occupiers of Afghanistan. Later, he encounters the Soviet officer who beat him in prison and kills him. The officer does not remember Assef, but Assef informs him that "I do not forget a face." Something of a borderline example in that the war pretty much fell into Black and Gray Morality by that point (in Assef's case, somewhere beyond black).
In Stormbreaker, the death of Alex's uncle and last living relative sets in motion the events that lead MI6 to recruit him as a secret agent. When Alex meets Yassen Gregorovich, the assassin who carried out the murder, Gregorovich merely shrugs and says 'I kill a lot of people.' However, he probably wasn't denying it so much as invoking A Million Is a Statistic, and knew exactly who Alex was talking about, both because it happened fairly recently and because in later books, its established he knew Alex's father personally and considered him a friend, and recognized Alex on sight. It's unlikely he didn't know who Alex's uncle was.
Sisterhood Series by Fern Michaels: Played with rather oddly in the book Sweet Revenge. Reporter Ted Robinson, who is not quite a hero at that point, asks Rosemary Hershey a question about two people. Rosemary says in a genuinely puzzled tone "Who?" Her soon-to-be ex-husband, Bobby Harcourt, pokes his head into the room and states that those are the two people she killed, along with their young daughter. She killed three people and she is the only one who apparently did not even bother to remember them. However, she ends up remembering the name of the young daughter - Diana. She actually blocked that out and presumably other details of her killing them. Once she remembers, however, she is unable to stop thinking about it!
Harry Potter and the Order of the Phoenix: Harry momentarily forgets that several books ago, Ginny was possessed by Lord Voldemort for an entire year. For Harry, this was just one bit of yet another adventure. For Ginny, it may well be the most significant event of her entire life. He says he forgot about it, and her reply is a cool "lucky you". Harry feels sincerely sorry and apologizes.
In an earlier book, Harry wakes up from a nightmare, and Seamus just casually asks if Harry's being attacked again.
During Oberyn Martell and Ser Gregor Clegane's fight, Oberyn reminds Gregor that he raped and murdered Elia Martell, Oberyn's sister, and killed her children. Gregor replies with "Who?" Thanks to Oberyn's constant, incessant reminders, Gregor eventually does remember, and before he kills Oberyn he acknowledges that he murdered Elia's chldren, then raped her, and then killed her. He wants Oberyn to know the order it happened. It's possible that Gregor just pretended not to remember to taunt Oberyn, given that this wasn't just some random atrocity out of the many he's committed, but the wife of Prince Rhaegar Targaryen and kind of important. Even Dumb Muscle like Gregor would probably remember something like that.
Tyrion used to have a wife, Tysha who truly loved him. Tywin convinced Tyrion that she was a paid whore only to break his spirit. Not only that, but he made his entire garrison violently gang-rape the poor 13-year old girl before Tyrion's eyes. And then forced Tyrion to participate as well. When Tyrion finds out the truth, he attacks Tywin, and holding him at crossbow-point, asks him about Tysha. Tywin doesn't remember the name, and when Tyrion tells him who Tysha was, Tywin can't recall what happened to her after the gang rape.
Subverted in The Reckoners Trilogy: David remembers every single detail of the day that Steelheart murdered his father. He's shocked to discover that so does Steelheart. After all, the reason he killed David's father was because the man managed to injure him, the first time that had ever happened. The event was just as important to him as it was to David.
The Professor: “Imagine if I walked into your home and started smashing up the furniture while demanding to know why you killed my dog. Even if you did kill the dog, how would you know what dog I was talking about?”
The Wheel of Time had the Aiel War. When King Laman of Cairhien Cut down the Avendoraldera, the tree that the mysterious desertdwelling Aiel-people had given his people as an sign of friendship hundreds of year earlier in order to carve himself a throne, four of the twelve Aiel Clans crossed the Dragonwall and invaded the Westlands. For the Westlands It was the greatest war they had seen in centuries, uniting almost every country under one banner that was stil unable to stop the Aiel horde. To the Aiel it was simply the execution of the Oathbreaker King Laman, and once they succeded in that they turned around, returning to their desert without giving any of the lands they had conqured a second glance.
In Six of Crows by Leigh Bardugo, protagonist Kaz's entire life has been shaped by the con gang boss Pekka Rollins pulled on him and his brother that resulted in his brother dying. A flashback reveals that less than a month after it happened, Rollins didn't even recognize Kaz when he tried to confront him.
This is briefly brought up in the Miss Marple mystery The Mirror Crack'd from Side to Side. Jason uses this explanation when asked about the "previous relationship" between Marina and Heather. He explains that Heather was a fan of his wife's, and as a result, it was a big deal for her to get Marina's autograph. However, Marina has done hundreds of receptions with fans and signed thousands of autographs, so she simply has no memory of the event Heather described of one more autograph seeker among thousands. Subverted, in that Jason is well aware that while Marina didn't remember Heather specifically, the earlier meeting was even more significant to her than it was to Heather.
Codex Alera: A heroic version. In the backstory, the late Princeps Septimus, sole heir to throne, was not only a powerful mage comparable to the reigning First Lord, his father, but a genuinely good man. He could not stand by and watch as abuses happened in the lands he would one day rule, from the abuse of the magically-stronger Citizens over the weaker commoners, to slavery, to the idea of marriage for simple political and genetic gains. He fought for the downtrodden, sought to end slavery, and rejects the arraigned marriage his father set up with a magically-strong and smart noblewoman in favor of wedding a commoner he has fallen for. The people he helps, and those who oppose him angered, are many, so he can't remember them all. They, however, do remember and enter into a cabal to kill him. It includes not only his spurned fiance, but two powerful High Lords, second in rank and status to the First Lord. They succeed.