In "Tales of Ba Sing Se", a clip is shown where Iroh is crying over the loss of his son. Before the episode was aired (but after the rest of the voice-overs for that season were completed), the voice actor for Iroh, Mako, died. As such, it was only appropriate that they include a tribute to Mako at the end of his story.
Iroh exasperatedly telling Zuko he'll never find the Avatar in the first episode. Once you know Zuko's whole backstory, it comes off pretty damn callous in a rewatch since he's offhandedly saying "Oh, just give up on ever returning to your homeland, family, and birthright and go to bed." To be fair, they had no legit indication at the time that it was anything more than a Snipe Hunt used to manipulate Zuko to keep him from turning face in his exile.
In the Guru, Azula taunts to the Earth King when he learns that Long Feng was manipulating him by saying "It's terrible when you can't trust the people who are closest to you." It become dramatically ironic when latter, her own friends Mai and Ty Lee turn against her which causes a big Villainous Breakdown.
Hama passing down bloodbending to Katara doesn't seem like a big deal until The Legend of Korra, where two of the series' main antagonists are bloodbenders, and they don't need a full moon to bloodbend.
Zuko blowing off Toph when she tries to tell him about her unresolved issues with her parents, followed by her feeling disappointed that she didn't find a resolution to this central issue for her, becomes much harsher when The Legend of Korra reveals that Toph not finding a resolution to her issue with her parents affects her parenting of her own future daughters, which creates a host of emotional issues with them regarding her parenting.
In-universe, "Nightmares And Daydreams." An episode where Mai is trying to cheer up Zuko, who just stumbled out of a war meeting with his father and sister, but he doesn't mention what happened in there. He seems depressed over it, though. We later see that this was the meeting where the Fire Nation decided to burn the entire Earth Kingdom to the ground, turning its surface to glass and murdering every single person there.
The second season of Avengers, Assemble! "The Arsenal" featured Stephen Collins as Howard Stark. The episode aired around the time TMZ leaked an audio recording of Collins admitting to now ex-wife Faye Grant that he had molested several underage children decades before.
One early Family Guy episode featured a gag where a child on Kids Say the Darndest Thingsdescribed being molested to Bill Cosby. Over a decade later Cosby was exposed as a serial sexual predator himself, with over fifty alleged victims.
The G.I. Joe: A Real American Hero episode titled "Cobra Quake" involves G.I. Joe trying to stop Cobra from causing an artificially created earthquake to destroy Tokyo. In 2011, an 9.0 magnitude earthquake actually hit Japan. Because of this, The Hub suspended airing this episode on its channel until further notice.
Little Rosey was one among many Celebrity Toons and Spin-Off Babies featuring a no doubt slightly unrealistic portrayal of Roseanne's childhood. Considering her later incest allegations and the fact that the character Buddy was based on Tom Arnold and the ugly, bitter end to their marriage, it's best to pretend this never happened — and not because it was a bad cartoon.
The "Bright Lights" series of My Little Pony 'n Friends featured a pop star, modeled after Michael Jackson, whose manager stole fillies shadows as a form of Applied Phlebotinum. Years later, Michael Jackson began to face accusations of taking indecent liberties with children.
The episode "Trade Ya": Twilight Sparkle decides not to sell her books, saying that just because they don't need to be read again doesn't mean they don't possess value; they're a part of her history and what made her who she was, and were quite precious. Four episodes later in "Twilight's Kingdom", they along with the rest of the Golden Oak Library were destroyed by Tirek.
The Season 1 episode "Owl's Well than Ends Well" involved Spike having an insane fit of jealousy over Twilight seemingly replacing him with an owl, cumulating in him running away because he felt she didn't love him anymore. The Season 3 premiere "The Crystal Empire" puts his actions in a new light, as we find out that Twilight no longer needing him is Spike's worst fear.
Season 1's "Griffon the Brush-Off" ended with Gilda ditching Rainbow Dash as her friend. Four seasons later, the events of the episode is re-explored, and we learn that griffons literally have no concept of friendship, putting everything involving Gilda in a much different light. Rainbow Dash is the only friend she's ever had, and she has little idea how to reconcile that with Rainbow having other friends. They become friends again in the end of the episode.
In Season 1's "Look Before You Sleep", Twilight says that she's never had a sleepover before and calls out Applejack and Rarity when their bickering almost ruins her first slumber party. The Season 5 episode "Amending Fences" puts the whole episode in a different perspective when we see that Twilight had friends in Canterlot and her context implies that she never had a sleepover with any of them. It becomes even worse when we see that Twilight leaving Canterlot left an emotional scar on her old friend Moondancer.
In the season one episode "Call of the Cutie", Scootaloo tells Diamond Tiara she's 'stuck being stuck up' as a Shut Up, Hannibal!. We find out much later that she's literally stuck, as her mother was browbeating and emotionally abusing her to be a stuck up Rich Bitch like her.
A lot of Over the Garden Wall. Wirt blames his brother for getting them lost because he feels that Greg's antics are what caused them to fall into the pond where they're both currently drowning. He eventually gives up on attempting to get home and lies down on the ground to sleep, i.e. loses the will to live.
Star Wars Rebels: In the tie-in comic "No Sympathy", Agent Kallus arrests two young and disillusioned Imperial officers, including a former student of his, because they tried to defect to the rebels. In "The Siege of Lothal", he and Darth Vader assassinate Minister Maketh Tua because she tried to defect, and frame the rebels for it. The events of "The Honourable Ones", however, wind up triggering a Heel–Face Turn for him, which ultimately leads to him becoming a Reverse Mole and later defecting from the Empire, with extremely strong parallels to what Tua and the two young officers tried to do. Which puts his earlier actions in a far harsher light than they already seemed, and it's entirely probable that he knows this.
In the movie, Anakin and Ahsoka rescue Jabba's child. In Return of the Jedi Jabba tries to have Luke (Anakin's son) executed twice, and has Leia (Anakin's daughter) chained to him in practically nothing, which ultimately backfires when she strangles him with her chains.
Episodes 15-18 of Season 4 are about Obi-Wan going undercover to uncover a Separatist plot against Chancellor Palpatine. On January 26th, the day before the second episode in the arc premiered, Ian Abercrombie (Palpatine's voice actor) passed away. Fortunately the voicework of Season 4 was already done, and so Abercrombie was able to finish voiceovers before he died.
Remember the Father's warning to Anakin when he refused? That his selfishness would haunt him and the galaxy at large for refusing to replace the Father? Although at the time the episode was made, they probably were intending to mean the oncoming Jedi Purge, but after the revelation of Abeloth's ties to the Ones in the final book of Fate of the Jedi: Apocalypse, as well as what Abeloth did throughout that series, the Jedi Purge actually seemed not so bad in comparison. This one, at least, is no longer canon.
In "The Deserter", Rex states that if the Republic loses the Clone Wars, then "our children and their children could be forced to live under an evil I can't well imagine". Little does Rex know, that such a fate will befall the galaxy when the Republic (or, more accurately the Empire) ''wins'' the Clone Wars, with the galaxy falling under Palpatine's control. Not helped by the fact that both the Canon and Legends timelines would turn the later victory against the Empire into a case of Happy Ending Override.
In "Monster", Savage Opress tells Ventress that "so long as I live, you will not harm him!", referring to his brother, Feral. Soon after, Savage was rather brutally proven right: Ventressdidn'tharm Feral. Savage himselfdid.
Ahsoka fighting a brainwashed Barriss in "Brain Invaders" becomes this after the season 5 finale, where Barriss is attacking Ahsoka of her own free will.
Also at the end of the episode Ahsoka wonders whether she should've killed Barriss (who in a moment of clarity, even asked her to do it) because her mercy could've potentially jeopardised the lives of many others. Barriss ends up killing at least seven Jedi, at least two civilians, and injuring many others with bombing the Jedi Temple.
On a similar note, Ahsoka mentioning Clone Sergeant Slick's betrayal to Barriss in the same episode also becomes this when Barriss betrays the Republic and Jedi Order for similar reasons to Slick.
Ahsoka's line in the pilot movie, "I'm a Jedi Knight! Or soon will be." In the season 5 finale, she rejects the possibility of knighthood and leaves the Jedi Order.
In "Clone Cadets", Bravo Squad's line "Time to watch the Dominoes fall" becomes this when you consider the ultimate fate of Domino Squad. Also in the same episode and deliberately invoked at the time of making, Hevy's promise of coming back to 99 for his medal. Considering that the episode was a prequel to the Season 1 episode "Rookies", fans who've watched the episode beforehand know that's not going to happen.
A lot of the series is inevitably going to be this in light of the events of Episode 3. The friendships we see developing between main Jedi characters and various clones makes Order 66 all the more tragic. In The Clone Wars Anakin is also gets to be a lot more personable and likeable than the movies that are centered around his downfall which makes his friendship with Obi-Wan much more believable. This only serves to increase the tragedy of said eventual fall. Of particular note is the friendship displayed between Anakin and Mace Windu, whereas in Revenge of the Sith, Mace reveals he never really trusted Anakin.
In "Weapons Factory", Luminara tells Anakin that she is prepared to let her go of her apprentice Barriss in the event of her death, and asks if Anakin is willing to do the same with Ahsoka. Come "The Wrong Jedi", both of them are forced to let go of their apprentices for reasons that neither of them expected.
Luminara Unduli being held prisoner by reanimated Geonosian warriors in "Legacy of Terror" becomes a lot more disturbing when you consider her fate in Star Wars Rebels.
The opening arc of Season Two becomes much, much harsher in hindsight after Season Two, Episode Eight of Rebels. Here, we see that the Inquisitors kidnap helpless babies with the intent of raising them in the ways of the Dark Side. And this time, the Jedi Order and the Republic aren't around to stop Palpatine's plans.
Ahsoka saves Tarkin in "Citadel Rescue" from being killed at the hands of a Separatist general. Guess what Tarkin would go on to do to Alderaan in A New Hope...
A mind controlled Ahsoka fighting Anakin in "Altar of Mortis" becomes harsher after the Rebels Season 2 finale, when she confronts Darth Vader.
Seeing R2-D2 and C-3PO run out of power and Threepio giving a eulogy-like speech to Artoo in "Nomad Droids" becomes much sadder after Kenny Baker's death.
One of the few times Steven snaps is when he yells at Lars for insulting his mom. "What do you know about my mom? I didn't even know my mom!" Fast forward to Rose's Scabbard and Pearl, in the midst of a breakdown, throws this right back at him. "What do you know? You've never even met her!" The similar wording especially makes the first instance so much harder to watch.
"On The Run" makes many of the events regarding Amethyst much more bitter. Especially one of the jokes made in "Steven's Lion".
Pearl: We're really going to let him keep that [lion]? Garnet: We kept Amethyst.
In "Three Gems and a Baby", Garnet gets very upset after Steven starts crying when she unfuses. Now rewatch "Jailbreak". Ruby screams at Steven not to look at her, and one of the first things that Garnet says to Steven after refusing is "I'm sorry, we didn't want you meeting us here like this". She clearly thinks Steven was upset because she's a fusion, but in reality, Steven just had No Object Permanence at that age, and didn't understand where Garnet went.
Steven is quite upset whenever something bad happens and apologizes to whoever he feels he can to for not having been able to help them, even if to an enemy. "Mindful Education" then reveals that he has crippling survivors guilt to the point where he blames himself even for the things that Rose had done. To the point where he almost commits suicide by falling to his death. And the worst part? If Connie hadn't been there, it might have ended in a major Downer Ending...
The Transformers: Rescue Bots episode "Tip of the Iceberg" contained what happened to be a heartwarming moment when Dr. Morocco handed the Burns family a painting of Chief Burns' grandfather. "Changes" revealed that that he didn't do it out of any kindness, but as a way to sneak in a spy camera so he could spy on the family and the Rescue Bots.
Season 4 of Ultimate Spider-Man features a lot of heartwarming moments with the Scarlet Spider where he learns to trust others and open up to Peter, his friends, and Aunt May—which became a lot less so in with "The New Sinister 6, Part 1", where it was revealed that he was really The Mole for Doctor Octopus, meaning those moments were really him playing Peter to make his betrayal hurt worse.
Kaeloo: In Episode 35, Kaeloo claims she's leaving and moving somewhere else as a result of Mr. Cat not confessing to something he did. Mr. Cat, not wanting her to leave, confesses, and then Kaeloo reveals that she was lying to get him to confess and then harshly punishes him for what he did. We later learn, in Episode 52, that Mr. Cat has already lost several loved ones, meaning that Kaeloo leaving in Episode 35 would have added to that number, which is probably why he confessed. And then he found out he was being emotionally manipulated and got punished for that.