In Super Mario Bros. Z, Mario stomped Captain Basilisx's best friend into a lava pit and used his shell to slaughter the rest, but Mario doesn't remember it. Although Fridge Brilliance sets in when you realize that there are no lava pits in the room that Basilisx mentions. Mario may not remember it because it may have never happened in the first place, to say nothing of the fact that the way he describes the death is literally impossible to pull off in-context. The only true fact about his story is that there was indeed a third Koopa Troopa behind Door #2 of Bowser's Castle in Dinosaur Land.
In Death Battle, this is the stated reason why Guts manages to defeat Nightmare in their battle. Guts has always fought impossible odds against terrifying demonic forces due to the brand on his neck attracting them like moths to a flame, and yet Guts has also always managed to come out on top if only through sheer dumb luck and unstoppable willpower; Nightmare and the Soul Edge really aren't anything he isn't already accustomed to.
The Most Popular Girls in School: In Episode 72, Mackenzie's idol, Jeannie Halverstad, doesn't even recognize her at first, despite clearly talking with her phone the phone the episode before, and having met her at Cheer Nationals many episodes before that.