Wolverine Vol. 2 issue #75: "SCHLUNK!" The sound of bone claws tearing through Wolverine's skin.
Wolverine in Uncanny X-Men 132: The Hellfire Club defeats the X-men, and Wolverine gets mass-increased until he crashes through the floor and winds up deep underground. He rises from the sewer, livid, and utters the line that changes him from secondary character into major hero and cultural icon...
Another Spider-Man example with a huge impact on the entire mythos and which comes as a revelation not just to the readers, but to the protagonist as well, from The Amazing Spider-Man (1963) #257:
Mary Jane: You don't have to make up another one of your phony excuses, Peter! Not now—! I know the truth! The real truth!
King in Black:Arc Villain Knull, god of darkness and creator of the symbiotes, has just staged his attack on Earth. He demands that "Brock", the greatest threat to him, be brought to him and executed, and then he can kill the planet quickly instead of dragging things out. Venom/Eddie Brock seals away his son, Dylan, in a bunker for safety, and heads to meet Knull...who realises there's been a misunderstanding.
Knull: I suppose I should have specified which Brock.
In a classic 1968 tale, Namor is on a rampage in New York, battling the Thing. Suddenly, a woman mostly in shadow and calling herself "Mrs. Prentiss" steps out. Her appearance shocks Namor to stop his battle although the woman says "what do we have to say to each other..now?" At home alone, Mrs. Prentiss writes in her journal as she reveals to the reader she was once a very special person in Namor's past.
Mrs. Prentiss: No one could have stopped him except a widow named Mrs. Prentiss...who the eternally young Sub-Mariner once knew as....Betty Dean.
Punisher: The Platoon: The story starts off with the author Michael Goodwin interviewing Frank's former subordinates about their experiences in Vietnam. A flashback shows Frank call in an airstrike against an abandoned village where enemy soldiers are waiting in ambush. the scene cuts to a survivor of the airstrike talking about what happened with a superior and swearing revenge. Then, the final panel of the issue shows the veterans looking stunned, and asking Goodwin, "How do you know this?", revealing that was something that the author just told them about. The next issue reveals the (fairly innocuous) answer: Goodwin also interviewed the NVA commander for his book, but at the moment the line is uttered, it's pretty startling and spooky.
Kirsten McDuffie: Will you state your name for the record? Matt Murdock: Absolutely. My name isDaredevil.
During #25 of Mark Waid's run, Matt fights with Bullseye's new enforcer, Ikari, who was splashed with the same chemical that blinded him and gave him his radar sense. Matt attempts to use his experience to his advantage. That officially ends with this:
The storyline "The Murdock Papers" revolves around the titular documents, which Kingpin states will prove Daredevil's identity and his culpability in various crimes. Four issues in, after Daredevil has blatantly broken the law to retrieve the papers:
Kingpin, to the police: I suppose I should be honest with you... there are no Murdock Papers.
Xavin: "Chase... ran away. I believe he had a disagreement with Nico about what to do with Gert's body."
Ultron: "Astounding. I anticipated your defeat of Doom, yet I was 99% certain you would be unable to damage him badly enough to uncover that he was merely one of my androids. Truly, you are my greatest creation."
Captain America and the Mighty Avengers Issue #5 ends with one accompanying a Wham Shot that brings the last thing anyone was expecting to the Marvel Universe:
One being about the time traveler Doctor Strange was wondering about.
Clea: [Stephen Strange] was wrong. [Virginia Dare] isn't the time traveller. Is she, Rojhaz? Rojhaz/Steve Rogers: Well, if you put it like that, Ms. Strange, I guess she's not.
The other is about the universe Steve's presence was altering before it was split off into a pocket universe
