In North Wind, Loki ends up delivering one of these. Given that Thor had just accused the God of Mischief of lying, he kind of walked into it.
Thor:[accusatorily] You are a vile liar! Loki:[still reading his book] You will have to be more exacting than that.
In the Pony POV Series, Discord reacts this way to recounting his multiple counts of genocide.
NightmareParadox has been pulling her "Groundhog Day" Loop revenge plan on Dark World!Discord for so long she's forgotten when she started, and reveals she doesn't bother remembering the names of anyone but her main pawns, especially the innocent bystanders who get killed, as shown when she doesn't remember who Poison Apple (Apple Pie's identical twin sister) is, asking if she's her coltfriend when asked how many times she's let her die.
Ditzy Doo's "Butterflies" story has a more humorous example with this exchange between Sparkler and Prince Blueblood (who's since undergone Character Development in his own story).
Prince Blueblood: You look rather familiar, we’ve met, haven’t we?
Sparkler: Uh…yeah…we had a date…It ended with me chewing you out.
Prince Blueblood: Unfortunately…that doesn’t narrow it down as much as I wish it did…
Referenced in the fanfic Naruto: Consequences once Naruto and Hinata find out that Konoha has been destroyed and that the Fire Daimyo committed suicide.
Naruto: For Fire Country, the destruction of the Leaf and the suicide of their leader will probably be the most important day of their lives.
Hinata: Really? So what was it for the both of us?
Naruto: For us...It was Tuesday.
Also referenced in Naruto: Wrath of the Emperor
Naruto: "You tried, remember?" "And what was the end result? Oh, that's right." He ticked off on his fingers. "Sixteen ANBU dead. Three maimed Jounin. One Sannin missing an arm. And Kakashi Hatake dead. And that was me and Hinata without trying. I am not the same gaki you once knew, Tsunade. You and all of Konoha should do well to remember that. The Akatsuki found out the hard way. You should be thanking me, Senju for eliminating the Akatsuki. Wiping them out was probably the most important event in the history of the Elemental Countries."
Spider-Man: "The Destruction of the Milky Way Galaxy... our Galaxy."
Johnny: "Huh... must be Thursday."
Spider-Man: "I never said I was afraid Agent Hill, nervous yeah. I mean here we are again, going up against impossible odds with a plan that may or may not work, oh and not to mention our planet, excuse me Our Galaxy is hanging in the balance."
In the Baldur's Gate fic Empty, Kivan finally confronts the ogre who tortured and murdered his lover, triumphantly announces that he will die for killing her, and is appalled to find that the guy has no idea what he's talking about.
In the Mass Effect fic Square One, A Cerberus scientist who had stolen research and had been on the run for years looking over his shoulder thinking they were after him finds out they forgot about him years ago until they finally needed the research back.
Miranda: "Poor little man." Miranda's voice was cold as ice as she held her submachine gun on him. "To you, the day you scraped up the guts to plant a knife in my back was the bravest and proudest day of your life. But to me? It was Tuesday. I didn't give a damn where you went, and I wouldn't now if you didn't have something I wanted."
In Harry Potter fic If Thems The Rules, after getting stabbed and threatened by Grindlewald Harry blithely goes about his day, putting it out of his mind.
Attelus Kaltos of Secret War has a subversion of this when a local Hammer confronts him for killing his brother. He doesn't remember it at all, after killing so many in the many months past. But is still genuinely sorry for the loss. For a short time at least.