JoJo's Bizarre Adventure Part 1: Zeppeli notes that while he doesn't know Dio personally, the Stone Mask that gave Dio his vampire powers is responsible for the death of his father. Zeppeli asks Dio how many lives he's sucked away to heal his wounds from his previous fight with Jonathan. Dio coolly asks Zeppeli in return if he remembers every piece of bread he's ever eaten. Cue It's Personal.
A (sort of) heroic example in Mugen in Samurai Champloo. On at least two occasions, when a psycho is shown harming Mugen's True Companions and pulling a No Doubt The Years Have Changed Me, Mugen responds that he has no idea who that person is, and has no recollection of the nasty thing he did to them. It's possible that Mugen actually does remember and acting this way was strategic, as it works effectively to distract the psychos from their Evil Gloating and upset them enough to make mistakes. Or Mugen just likes pissing people off.
When Ichigo comes face-to-face with the Hollow that killed his mother, Grand Fisher doesn't remember the incident at all until looking into Ichigo's memories.
Hilarious hero example. Ichigo can't remember a punk he beat up four years ago, who had to move town because his dad got transferred and was looking forward to a rematch.
The first time they met, Mask de Masculine easily knocked Renji across the city with a single kick. Renji Took a Level in Badass and faces him again. Mask doesn't recognize him at first, claiming he can't bother to remember every villain he defeats (Mask is crazy and thinks he's the hero and the Soul Reapers are the bad guys).
Similar to Ichigo, Might Guy of Naruto cannot remember his previous encounters with the highly-memorable Kisame Hoshigaki. Each time Kisame was eager for a rematch and remembered Guy, but all Guy could manage to piece together after ten minutes of battle was "You wield a sword!"
Even funnier; Kisame is a notorious international criminal and is near or at the top of the most wanted list of several villages, including Konoha. When he is first introduced the other Jounin recongize him on sight, despite never having met him before, due to how notorious he is (and the fact that he looks like a shark), and he should have been even more notorious after being identified as a member of the international terrorist group Akatsuki, and as partner of the yet more infamous Uchiha Itachi, whom Gai knows personally. Yet Gai runs into Kisame three times and nearly died the second, and he still doesn't remember him.
Although after Kisame takes his own life to prevent the heroes from interrogating him, Gai says that he is a true shinobi, and that he will never forget his name.
This may be simply another example of Gai's Bunny-Ears Lawyer personality. The second time that Gai fought Kisame, Kisame was actually using a special clone, not the his own body. So to Gai, the second time probably didn't count. On the other hand, Gai may actually be that ditzy - it's hard to tell.
During the War of the Best, two of the Newkamas challenge Hawkeye Mihawk, saying that he may have beaten them before, but they were stronger now. Mihawk takes them out effortlessly, telling them as he does so that he doesn't bother to remember every weakling he cuts down.
In a heroic example, Luffy has this towards some of his weaker early opponents: While he recoginzed Coby during their reunion, he had apparently forgotten who Helmeppo was, when he ran into Buggy again, his reaction (in contrast to Buggy's shocked expression) was merely "Ah, it's just Buggy", and he fails to recall details like Buggy's earlier weapon, the massively explosive Buggy Ball.
Also, Zoro in movie 2, with Pin Joker.
When the Straw Hats first encounter Iron-Mask Duval, they think he may be someone Sanji ticked off at the Baratie. Sanji admits that during those days he upset a lot of people, and couldn't possibly recall all of them.
That case is actually an aversion though, because none of them had in fact ever met Iron-Mask Duval before and his grudge against Sanji was over the fact that Sanji's poorly drawn wanted poster looked nothing like him, and almost identical to Duval.
In Rosario + Vampire, Ms. Marin's husband was killed by the siren Kanade Kamiya. The confrontation is as follows:
Ms. Marin: It's you, isn't it? You're the one who killed my husband in our inn four years ago.
Kanade: Four years ago? Hmm... Oh, yeah. That's about the time I got assigned to this branch office. They used to send me all over the place back then. I hated this little town. Place reeks of fish... So I killed a lot of people... It was just a diversion to take my mind off things. I can't be bothered to remember everyone I kill.
In Outlaw Star, Gene immediately recognizes the MacDougall brothers' ship as the one that fired on and killed his father. He confronts Ron MacDougall, who cannot remember the specific job, although he admits it's probable he did it since he's done so many similar jobs.
In Samurai Deeper Kyo, a minor villain who was disgraced by Kyo during the war had an opportunity for vengeance after years of training for the very moment. Cue crowning moment of awesome when Kyo pretends to remember to get the man's hopes up, only to respond with "I didn't remember who you were, I just realized what a dumbass you are."
Used in the first OVA...when Kurapika confronts one of the villains, he doesn't remember what he's accused of. You'd think murdering an entire clan in order to steal their eyeballs would stick out in your memory....
In the manga, he doesn't remember either, until having a difficult time with Kurapika stirs his memory, and he confesses they were skilled and hard to kill.
In GUN×SWORD, Ray confronts the Claw in episode 13 who previously killed his fiancee, Shino. The Claw is an extremely pleasant old man and tells Ray that he has no problem with Ray killing him, but first politely asks him why. Ray tells him of the murder, yet the Claw cannot remember any of it. When Ray summons Volkain (the mech Shino was working on) a few minutes later, the Claw finally remembers what Ray is talking about and basically replies that Ray's grievance/goal is trifling compared to his own, and so Ray will have to wait.
Detective Conan: When he sees Shinichi Kudo apparently alive when he was presumed dead, Man in Black Vodka asks his partner Gin about it, only to have Gin remark that he doesn't remember the names of everybody he kills.
Sort of heroic example in Black Lagoon. In the "El Baile de la Muerte" arc, Revy teams up with Shenhua, Sawyer the Cleaner, and Rotton the Wizard, all mercenaries who were previously enemies in the "Greenback Jane" arc, and all of whom Revy had left for dead. While Revy knows the first two, she has no recollection at all of shooting Rotton, a hapless Joke Character, nor does she even know who he is. This greatly amuses Sawyer and Shenhua.
It doesn't help that's Revy thinks he's "some kind of man whore".
Another Heroic Comedic Sociopath example: Haruhi Suzumiya is challenged to "a duel" by the Computer Club president in retribution for her cruel Kick the Dog moment against him way back when.....but by this point, Haruhi had honestly forgotten all about that.
A heroic example Played for Laughs: the former Corrupt Bureaucrat Yoki lost everything after being defeated by the Elric brothers, but when he encounters them, neither remembers him. There's then a whole Unreliable Voiceover in silent movie style where he complains about his travels, but the Elrics don't pay any attention to him (and neither does anyone else in the room), In this case, the trope is used to compress several episodes worth of Filler into a short scene. Everything Yoki complains about happened in the manga and the first anime, but was cut from Brotherhood to focus more on the main story arc. Barry the Chopper gets the same treatment.
Inverted by Kimblee, who remembers everyone he's ever killed and treats doing otherwise as unprofessional.
Barry The Chopper also inverts this trope the same way Kimblee does. He gets very upset when Falman purposely misdates one of his murders.
Heroic example in Shaman King. In his initial appearance Tokagero possesses Ryu and seeks revenge against Amidamaru for killing him. When he reveals himself he's horrified that Amidamaru doesn't remember him on sight, and further that explaining he was the leader of a bandit gang Amidamaru slaughtered just prompt the revelation that Amidamaru slaughtered hundred of bandit gangs and couldn't be bothered to remember them all.
In Ginga Densetsu Weed, Hiro reminds Kamakiri that he (Kamakiri) killed his father and took out one of his eyes. In response, the Irish Wolfhound retorts that he doesn't remember, thus starting a fight.
Rolo from Code Geass comments at one point that he can't remember how many people he's killed, comparing it to how most people don't remember how many times they've brushed their teeth.
In similar fashion, when Lelouch asks the Chinese Federation's leaders the Eunuch Generals how they can treat their own people so poorly, their response is "Do you feel bad for the toilet paper you wipe your ass with?" Which of course leads to an Engineered Public Confession.
In Weiß Kreuz, Aya has devoted his life to bringing down Reiji Takatori, the man who killed his parents and left his sister in a seemingly-irreversible coma. When he finally confronts his nemesis atop a burning skyscraper Takatori has no idea who he is, still less what the people Aya's going on about have to do with him.
Tengen Toppa Gurren Lagann does this in the manga, where Viral brushes away a death threat by Kittan by telling him that his destroyed village was only one of hundreds; understandably throwing Kittan into a bloodthirsty rage.
Done in Yu-Gi-Oh! 5Ds. When Jose tells the heroes that Yliaster has killed people in order to keep history on its course, Sherry asks if this means her parents were among them. Jose replies they can't be expected to remember everyone they kill specifically. (This leads to Sherry and Mizoguchi being on the receiving end of a Curb-Stomp Battle - not using cards, I might add - done to show just how dangerous Jose is.)
Myria hates the protagonists for something they did to her, but they have no idea what it is. Subverted/parodied in that Myria can't remember what it is until another character taunts her about it.
Happens again with Team Fallen Angels, though they manage to remember after a few hours. Heck, there's implied to be a lot of guys seeking revenge on the "heroes", none of whom they recognize.
Another heroic example in Ninja Scroll: The Series. The widow of a samurai has spent years Walking the Earth with her son, trying to find the man who killed her husband so she can avenge him. She confronts Jubei, who says he's killed so many people it's difficult to remember specific ones. He does eventually remember when she gives him some details, and vaguely recalls having killed a samurai over a petty dispute (he'd bumped into the man, who had taken offense). The woman is naturally shocked that her life was destroyed over something so meaningless.
In the "Mushroom Samba" episode of Cowboy Bebop, a minor one-shot character shows up in town while dragging a coffin behind him. He was planning on taking his revenge on a drug seller named Dominoe because his shrooms killed his brother by making him laugh to death. Dominoe doesn't remember who this guy is because his shrooms had already messed his own head up pretty good. Then the coffin gets smashed by a passing bus.
Cypha: *while smirking* However, rejoice. I remember now. The one who left no adult or children alive on that day at that place was, without a doubt, myself.
Subverted with the rest of the Huckebein and Touma. When he accuses them of being behind the massacre of his hometown, Fortis replies that while they have done plenty of similar massacres, that particular one definitely wasn't them. He double-checked. And besides, if it had been the Huckebein, they would have made sure to leave no survivors, and Touma survived.
A heroic example near the beginning of Magical Girl Lyrical Nanoha StrikerS: when Subaru gets emotional upon meeting Nanoha, Fate does not know what's up until she is reminded that she and Nanoha rescued Subaru and her sister from a burning airport a few years ago. Ever since then, Subaru idolized Nanoha and trained to join her rescue force. Averted in that Nanoha remembers, though.
This may be almost Shout-Out grade. The fire happened on April 29, 0071 according to the in-universe calendar, which appears to differ from Earth's only in the numbering of the years. So, depending on what year that was - while the day that Fate and Nanoha graced Subaru's family was the most important day of Subaru's life, for Fate, it may well have been Tuesday. (Other calculations suggest it was Sunday, though. In an earlier series, June 3, 0065 was a Saturday. The cell phones in use suggest that 0065 is around mid-200X on Earth; of those years, the only one with June 3 on a Saturday is Earth's 2006. April 29, 6 years later, is Sunday.)
At first Hakkai from Saiyuki doesn't remember Chin Yisou, the last of the thousand youkai he killed and the one who deliberately kick started his Karmic Transformation This is vaguely understandable as Hakkai was at the time consumed with grief over Kanan's suicide and Yisou returns as a shikigami, if you kill someone you don't normally expect them to come back, at least before Hazel arrives in the story.
In the Mahou Shoujo Isuka OVA, when Isuka confronts the Witch Hunter who raped and killed her sister, he flatly tells her that he doesn't bother to remember all of the Witches he's killed. This turns out to be a lie; he does remember, and said this just to get a rise out of her.
In Tiger & Bunny, Barnaby Brooks Jr. has spent twenty years trying to track down the man who murdered his parents. When he finally confronts the man responsible, not only does the criminal not remember it, he mocks Barnaby for expecting him to keep track.
Of course, he cannot remember killing Barnaby's parents, because he was in completely different place that day.
In Claymore, Clare's one mission in life is to kill Priscilla, who killed her mother-figure Teresea. The moment of her death is seared into Clare's memory and drives her to kill, kill, kill when faced with difficult opponents, especially when she finally faces Priscilla for the first time in many years. However, Priscilla doesn't seem to remember her at all, since she only familiarizes herself with Clare because she smells like Teresea, whom Priscilla is obsessed with. Furthermore, on that fateful day, Priscilla totally ignored Clare because she only killed those who weren't young girls.
My Bride Is a Mermaid: In Lunar's debut episode, she recalls her beef with Sun, during which Sun always beat her at singing contests when they were kids. Upon being reminded, Sun sheepishly admits that, with everything that had happened to her since then, she forgot about Lunar, much to the latter's annoyance.
In Pokémon Special, upon first meeting Team Rocket Admin Koga, Misty saw him forcing a Ryhorn into evolution, and she realizes that he was the one responsible for doing a similar experiment on her Gyarados.
Koga: You can't expect us to remember every Pokémon that we experiment on.
Kyou Kara Ore wa!! has many examples, as the protagonists have beaten a lot of people, but the most incredible example is the so-called Perizoma Mask: right before the start of the series Mitsuhashi gave him the nickname, made him the Butt Monkey and generally ruined his life (and, in the process, getting the idea of trying and stand out with blond hair and becoming a badass) before moving to Chiba, and when the poor Perizoma Mask finally tracks him down Mitsuhashi can't even remember him.
A friendlier version occurs in Azumanga Daioh, after Sakaki defeats Kagura at the sports fest. When the new school year comes around, Kagura joins Sakaki's class, eager to continue her one-sided rivalry. However, when she mentions the sports fest, all she gets is a blank stare, as Sakaki doesn't remember her at all.
In Mobile Suit Gundam 00, Nena Trinity kills Louise's entire family in a random act of terrorism. When confronted about this years later, Nena's response is utter bewilderment at what Louise is even talking about. And Louise kills her right afterward, so she never gets reminded.
High School Debut has a heroic, non-violent example: Haruna used to be on her middle-school softball team, and had a reputation as an unbeatable pitcher. A batter from an opposing team who lost a game to her deliberately seeks her out later for a rematch, and is appalled to learn that, not only does Haruna not remember her, but Haruna doesn't even play softball anymore.
The heroic example happens in Ranma ½ during Ryoga's introduction, and is Played for Laughs. Ryoga attacks Ranma, who briefly can't remember him. The reason for the feud is then seemingly revealed: Ranma had been cutting in front of Ryoga in the tuckshop line, and had then left an appointed battle with Ryoga when Ryoga was four days late due to his incredible ability to get lost. This turns out to be a partial red herring when Ryoga reveals he had followed Ranma to Jusenkyo seeking revenge, and received a particularly unpleasant curse due to the actions of Ranma and his father, this being his new main grudge against Ranma. Although at first Ryoga himself didn't know Ranma had actually been the one who knocked him in; he thought the girl who knocked him in was some stranger, but he blamed Ranma regardless.
In the Hakuōki OVA Sekkaroku, a ronin attacking Okita declares that he intends to avenge his master. Okita doesn't recognize the name or remember the incident at all.
Miki: So, who is this guy? Akihiko: You really don’t remember? You used to pick on him all the time. Miki: I don’t know this guy at all. Akihiko: It’s only been four years! Kankuro: How awful, Nya!
Soul Eater plays with this. Excalibur tells stories frequently throughout the series, beginning each with "It all began on a Tuesday... or was it Friday? Monday is also possible. Hmm... Saturday it must be." Each time, he cycles through each day of the week, finally deciding on a day that isn't the one he started with.
Averted in Attack on Titan. During their first meeting, Bertolt expressed curiosity over what Eren experienced when his Doomed Hometown was attacked by Titans. Five years later, after learning that Bertolt is the Colossal Titan, Eren confronts him and asks how he felt learning about being responsible for Eren's mother dying. Bertolt quietly responds that he felt sorry for Eren, and it later revealed to have genuine remorse over his actions.
Edward King in Green Blood readily admits he has so many enemies he can't possibly remember them all. He does, however, know who the protagonist Brad Burns is and, while he initially fails to recognize Brad's brother Luke when they're face-to-face he quickly picks up on it when told.
Oga in Beelzebub does this to Miki in the Saint Ishiyama arc, Oga saved Miki from a gang leader when they were kids, and then proceeded to beat the hell out of Miki the day before he moved out of town. After Oga moves schools to Saint Ishiyama Miki, remembering Oga, seeks a fight with him to show how strong he's grown. Oga has no clue who he is. By the end of the arc, however, it is revealed that Oga may in fact remember him. Either that or he just found out Miki's name.
Tetsuro: My name is Tetsuro Oshino and you killed my mother, you metal bastard!
Count Mecha: I kill a lot of mothers. Let me guess: you want vengeance, blah blah blah . . .
What makes this even worse, if that were possible, is that Count Mecha has forgotten Tetsuro and his mother despite having her corpse stuffed and mounted on his wall.
InuYasha: In episode 97 ("Kirara Come Home"), the group confronts a monkey demon, whom they believe to have killed and eaten the missing Kirara; said monkey simply asks them if he is really to be expected to remember everything he's eaten. It turns out he didn't eat Kirara: Myoga had taken her to run an errand, and Shippo had been asked to inform the others as such, but forgot to do so.