Early in the manga, Bulma asks Goku about his parents, and Goku answers that he doesn't remember. Depending which media of his origin story you preferred, Goku was sent to Earth out of pure luck (Bardock the Father of Goku) or his parents sent him to Earth for his protection, but the end result that he loses his memories of them (Dragon Ball Super: Broly). The latter is even harsher with his mother Gine's last words to him were "Don't forget us, Kakarot!".
Given what Dr. Gero did to avenge the fall of the Red Ribbon Army it makes Commander Red's real goal even more disturbing.
Oolong's off-comment about Bulma not having a pretty death does happen later on as she ends up one of the victims of Super Buu, being turned into chocolate and eaten alive.
Master Roshi's destruction of the moon to reverse Goku's Great Ape transformation becomes horrifying when one remembers that's where Goku dropped off Boss Rabbit and his goons, though supposedly Toriyama said in an interview that Boss Rabbit and his goons got off the moon somehow.
A lot of the manga's gags having Kid Goku often run around stark naked as well as the early Running Gag involving him patting other characters in the crotch because he couldn't tell their gender otherwise can be unsettling with the knowledge that Gerard Jones, who wrote the English adaptation of the manga for Viz, was arrested for possessing child pornography in 2016.
The first thing Bulma does upon meeting Goku is shoot at him. Come Dragon Ball Super, the Bulma of the alternate future was killed by a villain in Goku's body.
A franchise-wide example: Goku's kindly yet dimwitted nature stemmed from a head injury suffered as an infant. Early in March 2024, Akira Toriyama died of an acute subdural hematoma (caused by a head injury) at age 68.
Regarding Freeza and his racism towards the Saiyans:
Frieza's viewpoint about the Saiyans ("You do realize how hypocritical this is? Your outrage over the loss of innocent lives? The Saiyan race isn't exactly known for its mercy. Compared to you brutes, I'm a saint!") becomes somewhat more depressing when we learn that in Super, a certain evil Saiyan who looks exactly like Goku commits genocide on a multi-planetary scale.note While it is revealed that he is a full-blooded Saiyan, he is actually Zamasu in Goku's body. Then we have the eventual appearance of Broly in Dragon Ball Super: Broly ("The greatest enemy is a Saiyan" anyone?).
In some localizations of the original Dragon Ball, Goku had the nickname "Monkey Boy" given to him by his friends in reference to his monkey-like tail and characteristics. However, with the frequent use of the term "Monkey" as a racial slur towards Goku and the other Saiyans by Frieza and his men in DBZ, his friends' nickname sounds awfully racist in retrospect.
Guldo's ability to stop time is portrayed as mostly useless since it only works when he's holding his breath. He's out of shape which means he can only do it for a few seconds at most and he's the weakest of the Ginyu Force, being easily overpowered by Krillin and Gohan. Two years later, and in the same magazine no less, DIO would show just how much damage a time stopping ability could do in the hands of someone truly powerful. It later becomes this within the franchise with the introduction of Hit.
When Ginyu switches bodies with Goku, it is treated as humorous moment for the most part since Ginyu is laughably incompetent at using Goku's body to the point that Gohan and Krillin can beat him up, and he gets utterly stomp by Vegeta. Then comes Dragon Ball Super, where Goku Black shows exactly how dangerous Goku's power and abilities are in the wrong hands, especially with someone who knows how to add their own skills on top of Goku's powerset.
When Vegeta is telling Goku about how he's cast out because he's born with a low battle power and is therefore considered worthless by Saiyan's society. This becomes hasher when you take Minus, in which Goku was sent to Earth not because he was weak, but to save and protect him from Frieza. Goku is led to believe for the rest of his life and even his afterlife that he was thrown away when it's the furthest thing from the truth.
In the Japanese dub, the episode where Buu kills nearly the entire supporting cast by turning them into candy and eating them. Master Roshi is one of those killed, and it becomes a lot more tragic when you learn that his voice actor, Kohei Miyauchi, died shortly after the episode was completed and it marked the final time he voiced the character.
During the Buu Saga, there's a subplot involving a duo of snipers killing innocent people For the Evulz. One of the shooters is reluctant at first until the more sadistic of the duo talks him into it. This sounds eerily similar to the Beltway Sniper Attacks that happened in the Virginia and D.C. areas in October 2002. Adding to it was that the English dub of those episodes first aired on Toonami when the shooting spree was still going on.
Watching Future Trunks fighting to change his future and prevent its destruction becomes this when you find out what Super did to it...
The filler episode with Mr. Shu, the teacher who abuses and belittles Gohan hits this when his English voice actor, a teacher and principal in real life, had one of his former students claim he'd been abusive to his students and staff, around the time the episode originally aired in the U.S. no less.
Future Gohan being ultimately murdered by the Future Androids eerily became this when (adult) Gohan's voice actor in the Latin American Spanish dub, Luis Alfonso Mendoza, was murdered in a shooting in February 2020. The fact that Mendoza was shot to death can also make all the scenes during the Buu Saga in which Gohan, as the Great Saiyaman, had to stop thugs who were constantly shooting at him very cringeworthy.
The infamous outtake where Goku calls Gohan a "faggot" is harder to listen to due to Peacemaker (2022), where Peacemaker's father is a homophobe who neglects his son while saying the same slur and Shadow the Hedgehog, where Sean Schemmel also voiced an alien who neglects his son.
Guldo's ability to stop time was played for laughs as while dangerous, Guldo himself was treated as a joke. Then the Champa Arc comes in and we get Hit, a much more serious character who's also much stronger a fighter despite having a weaker time manipulation. Not helping was the fact that Vegeta, the same person who kills the former, gets effortlesslyworfed by the latter.
Similarly, when Ginyu switched bodies with Goku in Dragon Ball Z, it was treated as humorous moment for the most part since Ginyu was laughably incompetent at using Goku's body to the point that Gohan and Krillin could beat him up, and he got utterly stomped by Vegeta. Goku Black shows exactly how dangerous Goku's power and abilities are in the wrong hands, especially with someone who knows how to add their own skills on top of Goku's powerset.
In the dub, when King Kai is talking about Beerus, he calls him a powerful deity on the level of the Supreme Kai. Goku becomes interested and says that since he's a deity, that means he's good. Yeah....Zamasu was also a deity and he was anything but good, despite claiming otherwise.
Cabba's talk about how the Saiyans from Universe 6 save planets from conflict now feels sinister since it's been revealed that they were working with Frost on some of their missions.
Many fans wanted U7 to lose the tournament against U6 so the Earth would be moved to Universe 6 and have new adventures there. Goku even lampshaded that there would be no real problem if they lost since they would simply moved to another universe. However, if U7 had lost and the Earth was moved, then Future Trunks would have no allies to get help to fight Goku Black since he would have remained in Universe 7.
After the revelation that Goku Black is an alternative version of Present Zamasu who used the Super Dragon Balls to take Goku's body and then murdered Goku who was trapped in his original body, all the praise Goku Black heaps on Goku for being the strongest of the mighty warrior race and how much he admires Goku becomes extremely creepy and disturbing. His Super Saiyan Rosé transformation scene is especially unnerving since Goku Black tells Goku to rejoice, since through him, his body had reached the height of beauty.
During the Champa Saga, Universe 6's Supreme Kai and his attendant are weirded out with the fact that in Universe 7, mortals can casually chat with gods. Later in the Future Trunks Saga, when Goku casually approached Zamasu, a Kai of Universe 10, and attempted to chat and spar with him, he took serious issue with it and this eventually became his Start of Darkness.
When the 2015 Dragon Ball Z movie was announced, all fans got was the tagline: The worst wish in history and the beginning of despair. One of the most popular theories was that someone wished for Goku to become evil. While the wish in this case turned out to be Frieza's return, the Future Trunks Saga does depict the creation of an evil Goku as the worst wish in history, which has brought true despair. The birth of Goku Black was the beginning of the end for Future Trunks' timeline.
The entire Future Trunks Saga becomes this once you see the ending. Future Trunks' entire timeline is destroyed by Future Zeno and that is after Fusion Zamasu kills everyone after losing his physical body. So despite beating Zamasu and going through hell, Trunks still loses everything except Future Mai.
Related to the Future Trunks Saga; during the Universal Survival arc, try watching Goku's friendship with Gowasu knowing that Goku and his friends will ultimately be the ones to cause Universe 10's complete destruction.
This Dorkly comic on how Dragon Ball GT could have been better includes a panel of Pan as depicted in GT dead, stating that she died of a heart virus similar to her grandfather in the future timeline. Now in a "slice of life" episode, Pan (as a baby) gets a fever and because of her Saiyan blood, the fever was rendered incurable. If they didn't wish Pan to be better with the Dragon Balls, she could've actually died in canon.
Bulma's parting lines in Episode 96 before everyone left for the Tournament of Power became more sad in retrospective after Bulma's Japanese voice actress Hiromi Tsuru passed away in November 16th, 2017.
Episode 105, "A Desperate Battle! Master Roshi's Sacrifice!!", in which the titular character almost dies after a spectacular show of force, became depressingly ironic and doubly emotional in the Hungarian dub, as Roshi's voice actor Ferenc Végh suddenly passed away after recording, leaving fans wondering for a month what his last episode would be.
Dub-specific: the episode where Vegeta said the following gem aired on the same night Mobile Suit Gundam: Iron-Blooded Orphans had its finale (where Tekkaden was routed and Rustal Elion got to build his utopia by having hunted down Tekkaden) air stateside (spoiler marking due to the implications):
on how Dragon Ball GT could have been better includes a panel of Pan as depicted in GT dead, stating that she died of a heart virus similar to her grandfather in the future timeline. Now in a "slice of life" episode, Pan (as a baby) gets a fever and because of her Saiyan blood, the fever was rendered incurable. If they didn't wish Pan to be better with the Dragon Balls, she could've actually died in canon.