This reveals that Red Hood is in fact a former ally; Jason Todd.
In the first season finale of Beast Wars, Optimus is piloting a stasis pod set to self-destruct towards this big planet-destroyer. The plan is for him to blow the doors and zoom out before the explosion.
Optimus: "All right, it's time. Computer, prepare to blow the pod doors."
Computer: "Negative. Pod doors are magna-sealed."
Megatron's voice comes out of nowhere over the speaker: "Oh. I assure you. It is not." Moments later... BOOM!
"Coming of the Fuzors, Part 2" sees the Maximals about to be destroyed by the Predacons. Cue a Big Damn Heroes moment from an unexpected source.
Gideon: It's not just about the Pines! I want that Shack! The physical building!
Gideon's Father: But why?
Gideon: Because it holds a secret you couldn't possibly imagine.
Explanation: Up until then, we thought that Gideon just wanted the Shack in order to get Mabel to be his girlfriend. We had no idea that he had other reasons for wanting the shack, reasons that extended beyond revenge on the Pines.
From the pilot episode:
Head Gnome: We'll never forget you, Mabel. ...Because we're going to kidnap you.
From "Gideon Rises":
Grunkle Stan: After all these years... finally, we have them all.
Hal: Show star systems destroyed by Aya. Show uninhabited star systems. Overlay.
Explantion: The two maps overlay, showing every star system Aya's destroyed to be uninhabited. Up until now Aya has been acting every bit a cold and emotionless robot, due to having turned off her emotions prior to her Face-Heel Turn. This is one of the key moments that shows that she's still got some of her humanity.
From the first episode of the four-part series finale: Zuko reveals that his father's plan isn't to cement his hold on the Earth Kingdom, it's to use the power of Sozin's Comet to burn the entire continent to the ground.
Hama: Congratulations Katara, You're a blood-bender.
Throughout "Lesson Zero," Twilight is terrified that Princess Celestia will punish her should she fail to complete a friendship report by the end of the day. Those familiar with the characters know full well that Twilight is highly prone to overreaction, particularly when it comes to both academics and the princess - who is nothing if not kind, understanding, and fun-loving. Thus, most people watching will expect Celestia not to care about the tardy homework assignment. This turns out to be correct, but Celestia isfuriouswhen she sees the mayhem that Twilight's reckless actions have wreaked upon the town, and the timing of her appearance comes as a huge surprise.
In "Speed Demon", the girls accidentally travel 50 years into the future and find that Townsville had been taken over by HIM. They find an elderly Miss Keane muttering to herself "I just stood there waving goodbye and they raced off and vanished for fifty years."
Heffer: Of course he will! It's like clockwork! Ladies and gentlemen, you're about to witness one of the seven wonders of the world. At about 11:30 PM, EST, through this window, our buddy Rocko will descend the staircase as he does every night for a glass of milk.
Filburt: And he ain't dressed for the occasion, if ya know what I mean.
Heffer: Yup. You heard right. He's completely...naaakeeed!
Madame Web: One day, you will yearn for the wisdom of Madame Web, and that day will come soon when the two-headed monster rises from the netherworld.
From the finale of the fifth and final season Star Wars: The Clone Wars. Ahsoka has been proven innocent in the nick of time. The Jedi Council are apologizing for putting her through Hell and are offering her a position as a Knight. Anakin offers her braid, symbolically welcoming her back to the Jedi Order. And...
In Teen Titans, the Story Arc of the first season was trying to stop the mysterious villain Slade from destroying the city, but once Robin finally gets to confront Slade and take away the trigger to his Doomsday Device, Slade reveals that the whole "destroying the city" thing was just a ruse on his part.
Slade: Trigger? There is no trigger, because there is no detonator.
Even earlier than that, when the audience learns Red X's identity in Masks:
Robin: It's about time we came face to face.
And in "Birthmark":
Slade: You're going to destroy the world, Raven...note Learning that one of the five main heroes is actually destined to bring about the apocalypse- and that the apocalypse is coming soon- is a pretty big Wham, especially since it provides the final piece to make all the bad things that happened across the episode click
Kraang: This did not prevent the planned escape of the human known as Kirby O'Neil.
Explanation: The episode this line was in focused around Donatello rescuing Mr. O'Neil after receiving a coded message from him. The escape succeeds, and this line comes at the end of the episode, turning everything the Turtles have accomplished on its head.
Shredder: That...is where you are you wrong. You took something from me...so I took something from you. Your Daughter.
Explanation: Shredder is talking about Miwa Hamato, who Splinter thought died fifteen years ago. Splinter Never Found the Body, but this is the first time in show that's flat out stating she isn't dead.
Karai: I have no honor... but that's about to change.
Explanation: After Leo tells her that her true father is Splinter, Karai began to have doubts about the Shreader's motives. It isn't until meeting with Splinter that she discovers that it was all true, causing her to turn her back on the Foot Clan.
Pumyra: "Lion-O, you are my king, but henote Mumm-Rais my master."
Explanation:Pumyra has, up to this point, been an ally of Lion-O and one of the good guys. During this scene, Lion-O asks her to throw him a magic stone. She instead hands it to Mumm-Ra. It drastically changes everything.
Red Tornado: Unfortunately, the Roy Harper we have all known for the last three years is another Cadmus clone.
And from "Happy New Year", FIVE YEARS LATER
Bart Allen: "Better get into character."
Explanation: The happy go lucky, carefree Bart from this episode is almost entirely an act hiding that he's from a Bad Future where The Flash is dead.
Subverted nicely, in "Depths".
Nightwing: She's dead.
Nightwing: We'd laugh about this some day.
For context, Nightwing asks Artemis to rejoin the team. On the mission, they fight Aqualad who has already been revealed to have made a Face-Heel Turn, who stabs her in the chest with the line "Welcome Back." After the second wham line, it's revealed to have been a ruse and Artemis is in fact alive, and Aqualad never really made a Face-Heel Turn
From The Hunt:
G. Gordon Godfrey: So, please explain to the good folks at home how it squares with the lies you told us, and the fleet of warships you hid beneath our oceans.
Explanation: Said to the Reach Embassador, whom up til now Godfrey has been completely supportive of.
Kaldur: It's too late, father. I've already... won.
Kaldur: That's all I needed to hear.
The Scarab: The Kid Flash's slower speed is making him a target for the energy blasts. In sixteen seconds, he will cease.
Jaime: Cease? Cease what?
In the Ultimate Spider-Man episode "Stan By Me", Spider-Man finds out that the Lizard is building a machine which Spidey believes that the Connors part of the Lizard built in order to drive out the Lizard. So, Spidey and company subdue the Lizard and use the machine on him. However...
Spider-Man: Why isn't it working?
Lizard: Working good. Erase Connors. Connors no more. Lizard forever!