Redcloak: Yeah, but you're forgetting that Greenhilt has some...thing...about his father, I think? Crap.
Even when Evil!Vaarsuvius teleports into his throne room for a duel, Xykon doesn't bother asking why he's there, and reacts to it more like "Oh, look. Another one."
Jirix: What the hell is going on!?
Redcloak: What, this? You'd be surprised how many people pop in to kill you when you're doing an evil scheme. Remind me when this is over: I'll tell you about the time, back in the day, a dozen druids jumped out of our potted fern.
Thief: Turns out the Dark Warriors and the fiends want us dead.
Red Mage: What?
Thief: Yeah, I don't get it either. Something about revenge for ruining their lives forever.
Black Mage: Oh, we've ruined millions of lives. They're just being babies about it.
Black Mage (and the entire group) get another example when Sarda claims to be Onion Kid, whose life Black Mage had ruined numerous times.
Sarda: But I started out as this young man! [Beat] Red Mage: I don't know who that is. Thief: All humans look alike. Which is to say, ugly. Sarda: That is a child who watched you slaughter his parents. Black Mage: Gonna have to be more specific. Sarda: After you broke his mind with a glimpse into the dread lattice of black magic, you killed his foster family. Then his other foster family. Then you destroyed his orphanage. Black Mage: Like, was this recently? Last week? A month ago? Help a guy out, will ya.
Prince Glitterbranch: This is about revenge. Arachne: I guessed that much. More info please. Prince Glitterbranch: You jilted me. Ignored my advances! Arachne: Uh...more? Prince Glitterbranch: You seduced and corrupted my own sister! Right in front of me! Arachne: Uh...nope. We need to narrow the field. Prince Glitterbranch: On the surface? In the Forest? I was A MAN! Arachne: Look. A girl's got hobbies. Was there anything unique about it?
Bun-Bun in Sluggy Freelance has killed a lot of people, so this trope was bound to show up eventually. In one of the Halloween arcs, the spirit of Christmas Elf Mr. Squeaky-Bobo comes back from the Dimension of Pain and, upon encountering his murderer, angrily tells him he is next on the list. Bun-Bun not only doesn't remember him, but mistakes him for a Neebler Elf instead.
Klaus Wulfenbach has damaged so many lives that when he is unconscious in a hospital, Gil has to defend him against dozens of assassin attempts, the first of which sets a precedent for the unimportance of motive (as seen in this elegant and finely-crafted link).
A particular favorite from the endless stream, below:
Insect Clank: DEATH TO THE DESPOILER OF EAST KRUMINNEY!! Dr. Sun: ...East what? Gil:Not really important! Dr. Sun: I suppose not.
Once in Scary Go Round, two gentlemen were talking. One asks the other if he has any (illegitimate) children; the other responds to the effect that a bear may not know how many insects he tramples as he treads through the forest, he knows only that there were many. Creepy.
Arthur and Sedrick from Wiglaf and Mordred have no memory of Gawain at all—despite having run into him on at least eight occasions—all of which resulted in him being shot in the head. So in other words, Sedrick has killed enough people that he hasn't noticed he's "killed" the same person eight times.
Invoked in Goblins when Thaco finally gets his revenge on the Goblin Slayer, informing him that he'll consider this battle a "random encounter" and that there will be no stories or legends told about it. This infuriates the villain, who considered himself a legendary enemy of goblins, greatly. Just to rub salt in the wound, he survives that battle only to die as an actual random encounter against a human fighter who's never heard of him.
In Muertitoshere: "Which Ursula Cowznofski whose life I ruined? 'Cause there's, like, four."
Wonderella: Who the Christ are you? Devlin: Er, I'm Devlin! You drove us off a cliff and nearly killed me last year, remember? Hell, we got married two years ago! Wonderella: I marry or nearly kill lots of guys. You'll have to be more specific.
Leo Modesto's Nuzlocke challenge of Touhoumon has this with Babs and the rival's Patchouli, who was responsible for killing Bab's partner and best friend. Given the nature of Nuzlocke challenges, it doesn't end well.
Inverted with Dubious Company's Sal. She is kidnapped so often that she treats them like filling out tedious paperwork and interrupts the kidnappers with small talk or criticism, much to their annoyance or disappointment. Heck, Izor's sacrificial-lamb gambit and some guy that had a crush on her cousin are the only kidnappings she distinctly remembers.