Much of the drama in the anime and manga of Dragon Ball comes from the characters constantly making bad decisions, being clueless about their own idiocy and then being punished for it.
In the Dead Zone movie, the villian Garlic Jr. has successfully gained Complete Immortality and is now immune to death - a point he spends the first half of the movie demonstrating. Finally, he has the heroes where he wants them and... You'd expect: He, knowing how dangerous the Z-Fighters are and that they defeated (at this point in the series) an entire army and Demon King Piccolo, a.k.a. the evil version of Kami, the guy who defeated Garlic's father despite overwhelming odds, kill them immediately. Instead: ...he opens up a portal to the titular Dead Zone to send them in. As in the place that is completely unescapable by anyone, immortal or otherwise. No points for guessing who ends up falling in.
In the Saiyan saga Goku splits a magic bean in two parts, so that Gohan and Kuririn can both have their part. They are fully recovered, even despite eating just half a bean. You'd expect: That from that point on they'd actually split any magic bean into two so that they have double the limited supplies of magic beans without having any drawbacks. Instead: No one splits a magic bean ever again...
During his fight with Vegeta, Vegeta transforms into his giant ape form in order to destroy Goku. Goku manages to use the Taiyoken ability to successfully blind him. You'd Expect: That the first thing Goku does would be to slice Vegeta's tail off and restore him to his normal form before he begins forming the Genki Dama in an attempt to finish him off. At least this way he would be weaker and he would be harder to find as he forms it. Instead: He just moves a couple feet away and immediately starts forming the Genki Dama all while Vegeta stays in his giant ape body. Then: Vegeta eventually recovers his vision and fires off a massive blast at Goku right as he finishes gathering energy, taking him out and making all the energy gathering he did worth nothing (or at least if Krillin hadn't arrived at the last minute).
According to Word of God, Krillin's Kienzan technique is capable of cutting throughanything (the only times it failed to cut through its targets were in anime filler scenes). Now, let's look at a certain point in the battle with the Ginyu Force: Recoome is about to use his Recoome Eraser Gun on Vegeta, while Krillin moves up for a sneak attack on him. You'd expect Krillin to bust out a surprise-attack Kienzan, slicing Recoome in half while he was busy with the attack and eliminating yet another member of the Ginyu Force, as well as making some headway toward not being regarded as a joke by the fans. Instead: Krillin slams Recoome's mouth closed mid-beam, accomplishing little more than knocking out his teeth and severely annoying him.
Later on, Krillin has just blinded Frieza using the Taiyoken. You'd Expect: Krillin would take this opportunity to use a pointblank Kienzan on his blinded enemy, ridding the universe of a great evil once and for all. It's not like he could have forgotten that he has that technique, considering he was attacking Frieza with it several times in a row mere seconds earlier. At the very least, you'd have a partially out-of-it Frieza who had to take time to dodge and could then be taken advantage of by someone else's attack. Instead: Krillin shouts at Vegeta to take the opportunity to blast Frieza... even though moments earlier Vegeta blasted Frieza with all his strength to no effect. Krillin is actually surprised when Vegeta doesn't take the opportunity. Let's see... technique that is guaranteed to accomplish nothing versus technique that could be a one-hit kill, regardless of the user's Power Level... Gotta go with the flashier one.
Related, there have been many, many occasions when Krillin has been watching his friends be knocked around by an opponent. Nappa, Vegeta, Recoome, Frieza, the Androids and Cell all instantly come to mind. During all of those fights he had access to both the blinding Taiyoken technique and the cut-anything Kienzan technique. You'd expect: Krillin would take the opportunity his friends are providing by being knocked around to sneak up, use the Taiyoken technique and then cut them in half with the Kienzan. Instead: Krillin just stands there and watches, not even bothering to intervene on one on one battles when he has no reason to not gang up on the enemy. During their fights with the Androids he doesn't even bother to fight.
While the gang is trying to survive Freeza, Goku is currently undergoing a healing treatment on his ship, but the rate of recovery is going very slowly. Krillin and Gohan have just discovered Dende's healing ability after he saves them both from otherwise fatal wounds. You'd expect: Gohan and/or Krillin try to sneak Dende away to where Goku is so Dende can heal him instantly as well and he can join the battle while Freeza is either distracted by Vegeta or Piccolo or focusing on transforming into one of his more powerful forms, or even just let Dende head to Goku on his own since Freeza doesn't know about Dende's healing power and considers him "small fry". Instead: Nobody thinks of doing this. Freeza eventually finds out about Dende's healing ability and quickly kills him to keep him from healing anyone else, and by the time Goku has finally recovered Vegeta is near death and the others are exhausted and worn out.
In the Frieza battle, when Goku begins forming a spirit bomb to take out the villain, Frieza catches him doing it, but to Frieza it looks like Goku is just standing there with his arms raised. You'd Expect: Frieza to attack Goku while he's defenseless and doing nothing, since it's perfectly within his nature, or at the very least look up - you know, the direction Goku's arms are pointing? Instead: He stands there staring at Goku for a good few minutes, trying to figure out what he's doing. Eventually he comes to the conclusion that he's given up, and, again, rather than attacking Goku, he simply flies off so the other heroes can distract him a bit longer.
The Z Warriors are fighting Androids 19 and 20. Bulma is however upset about not being involved in the action like before. You'd expect: She'd invest in long range surveillance equipment so she can see the battle from a safe distance, especially since she has a baby now. Instead: She flies right into the battle, giving 20 the distraction needed to escape and activate the androids that Trunks did know about. What's worse: Vegeta destroyed 19 with his Super Saiyan form and Piccolo had 20 ready for the kill when Trunks comes back from the future and tries to explain that 19 and 20 aren't the androids he warned them about. If Bulma didn't show up, 20 would likely have been killed before he could reach his lab, the other androids might've been tracked down and destroyed, preventing Cell from transforming at all.
The most extreme turning point of the Cell/Android Saga was only ever possible due to a massive avalanche of dumbassery on the part of damn near everyone involved.
Cell interrupts Android 17 and Piccolo's much-lauded even battle, which Androids 16 and 18 are watching in the background, and makes it perfectly clear that he wants to absorb 17 and 18 to achieve his perfect form. When Cell nearly kills Piccolo and utterly curbstomps 17, 16 steps in and stops Cell before he can absorb 17, then proceeds to more than hold his own against the bio-android. You'd Expect: For the Twins to immediately take the opportunity to RUN FOR THEIR LIVES. Instead: They stand and watch. What's worse: After 16 fires his Hell's Flash blasters on Cell, 17 ignores 16's warnings that Cell isn't dead yet and comes back into the battlefield area gloating about how the three of them can use the world as a playground. Cue 17 being absorbed and 16 rendered nearly nonoperational by Semi-Perfect Cell.
Follow that up with a series of events which involves Tienshinhan putting his own life on the line with a furious assault of Neo Kikoho attacks to keep Cell back, and Cell destroying an entire fleet of islands because he can't find 18. You'd Expect: 18 to take this opportunities and get AS FAR AWAY AS SHE POSSIBLY CAN, even if she has to abandon 16 to do so—in which case she should trust that Cell can't sense him either and wouldn't care to even target him. Instead: She only gets as far as hiding on one of the islands with 16. Only by fortune is her island the last one left that he doesn't destroy.
Meanwhile, Goku is up on the Lookout with Gohan, and both are able to sense everything that is going on, yet Goku knows that neither of them has gained the strength to take on Cell and survive. You'd Expect: For Goku to use the Instant Transmission technique to get his allies off of the island before their lives are on the line, and grab 18 while he's at it so Cell can't absorb her. Instead: Goku sits on his butt, doing nothing but making horrified expressions and explaining to Gohan that there's nothing they can do, hits Gohan to stop him from going to help Piccolo, only rescues his allies when they're both on the brink of death, and doesn't even think to grab 18.
Krillin arrives on the battlefield near 16 and 18's hiding place with an emergency suspension controller created by Bulma to shut off the cyborg twins so that they can be destroyed easily. By this point he's most likely sensed Cell's spike on power and figured out that Cell got 17. Despite his growing crush on Android 18 and his having discerned that the twins aren't evil incarnate, he knows that he probably has to do this in order to make sure Cell can't absorb 18 and become a full-on intergalactic menace. Also worth noting is that at this point Vegeta has arrived on the battlefield an Ascended Super Saiyan and is laying a physical and verbal smack-down on Semi-Perfect Cell. You'd Expect: Well, first off, for 18 to have FINALLY MADE A BREAK FOR IT BY NOW so that she wouldn't even BE THERE for Krillin to find. Barring that, for Krillin to be able to get a grip, immediately go to 18, convince her to leave and then lead her the hell away from the place, tell her about the suspension controller and make sure to keep it just in case his best efforts to save her fail and he has to use it anyway. Instead: 18 is STILL hiding practically right underneath Cell, gawking at everything as it goes on, even amidst 16's own warnings to run away and forget about him. As for Krillin, he spends a good 10-plus minutes lollygagging about his crush on her, makes the decision to destroy the controller, THEN comes over and tells her to run. It is at this point that Cell finally spots 18.
The crowning achievement of stupidity, however, has to go to Vegeta, who, again, while effortlessly curb-stomping Semi-Perfect Cell, acknowledges Cell's last transformation. You'd expect: That he would also acknowledge his son Trunks' warnings, considering that he knows a great deal about Cell, and annihilate the evil android while he still had the chance and prevent the destruction of the world as well as thousands of lives. Instead: Vegeta, despite seeing immediately that Cell's appeal to his massive overconfidence and lust for battle is a pathetically blatant ploy, inevitably gives in and agrees to allow Cell to attain his perfect form, which allows Cell the time to even spot 18 in the first place, then attacks his own son Trunks to get him out of the way, thinking only about getting a decent fight from the whole deal.
The result of all this (plus a Solar Flare): Cell absorbs Android 18, transforms into his Perfect Form, and proceeds to kick the crap out of Vegeta, Trunks, Krillin, and Android 16 while treating them all as a warmup, now capable of threatening the entire worldsolar system with his power. Nice Job Breaking It, Heroes!!
The Full-Power Super Saiyan means that by remaining transformed as a SSJ for extended periods of time the users can control their energy consumption much better than any other form of SSJ, thus they become stronger in training than any other form of SSJ would make them. While the FPSSJ users Goku and Gohan exit the training room in this state Vegeta and Trunks notice how genius this kind of training is, Vegeta even comments about this. You'd expect: That during their second year-long-training Trunks and Vegeta entered they'd be SSJ all the time, thus not only becoming FPSSJ but they'd be if not stronger at least equally strong as Gohan and Goku. Instead: Vegeta and Trunks completely forget about the benefits of training as a SSJ all the time, thus even though they trained double the time Goku and Gohan did, they were far weaker.
In the Cell Arc, when the Z Fighters are fighting against Cell, and Goku wants Gohan to fight. He thinks of giving Cell a Senzu Bean to insure that the fight will be "fair" because he didn't want Gohan to win without it feeling like a real victory. You'd Expect: For Goku to realize that fair fights do not matter in situations when an Omnicidal Maniac is trying to destroy the universe, and let Gohan fight Cell at his weakest instead, or better yet, Goku decides to fight Cell himself. Instead:He foolishly gives Cell a senzu bean!!! Really Goku, who really cares about being fair in situations like this??
Near the end of the Cell games, when Cell is very close to being defeated, he decides to self-destruct, taking the whole planet with him as one last act of spite. Goku decides to use instant transmission to teleport Cell away from the planet. You'd Expect: Goku to immediately teleport Cell into space, or anywhere free of life, instead of waiting until there was five seconds left of the countdown. However, it should be noted that Goku said he could only teleport to people/places he knew about beforehand. Instead: Goku teleports him to King Kai's planet with five seconds to spare, so that Earth would be spared. There is then, due to narrative decompression, a five minute window before Cell explodes. Also: Cell was knocked back into his Semi-Perfect form when he decided to self-destruct. Goku, Vegeta, and Trunks could've just jumped into the fight and killed Cell themselves. Goku could even have grabbed Cell once he powered down, teleported him to King Kai's planet, kill the monster in Other World, and teleport back to Earth. You'd Expect: That Goku, able to move quickly in the very least, would grab King Kai, Bubbles and Gregory and teleport somewhere safe. Instead: He stands there, looking dumb and saying something to the effect of "I'm sorry King Kai, this was the only place I could think of". The four of them are subsequently killed.
Related to that, just minutes ago before Cell activated the bomb in his body, Gohan had the chance to finish him off. Cell was badly wounded and couldn't regenerate, which would mean another attack would be enough to actually finish him off for good. Goku urgently yells at his son to do so now, as if the battle gets too drawn out Cell could grow desperate and try something really nasty. You'd expect: For Gohan to listen to his father and finish things immediately. Plus, it's not like he and his father haven't dealt with something like that before, as was the case with Frieza when he started growing desperate during his fight with Super Saiyan Goku, which of course led to Namek's destruction. Goku is actually speaking from firsthand experience. Instead: Gohan smugly feels Cell's suffering needs to go on and flat out ignores Goku's warnings, allowing him to restore his body. Eventually during the battle Cell is reduced to his Semi-Perfect Form, and Gohan still doesn't pounce on the chance to finish him off—which is what causes and allows him to trigger said bomb.
In the episode "Videl Is Crushed," Videl is getting badly beaten by Spopovitch. Gohan rushes to give her advice about giving up and how she isn't able to defeat Spopovitch. You'd Expect: Videl to take Gohan's advice and save herself from possibly dying. Instead: Videl continues to fight Spopovitch and he almost kills her by stepping on her skull, attempting to crush it. Thankfully: Gohan intervenes and Spopovitch was only forced to kick her out of the ring.
During the Majin Buu Saga, Goku faces off against a Not Brainwashed Majin Vegeta. He manages to punch him down and when he gets up, it seems that he's back to normal as he asks for a Senzu Bean in order to stop Majin Buu. You'd Expect: Goku should be suspicious that Vegeta might be pulling a trick to attack him while his guard's down. Instead: He does his request and looks into his bag for it. At that point, Vegeta hits him from behind, knocking him out in the process.
In Broly: Second Coming, Gohan arrives on the scene to find Goten, Trunks, and Videl getting the shit beaten out of them by Broly. You'd Expect: Gohan to go Super Saiyan 2 and own Broly’s ass. Instead: He only goes Super Saiyan 1 and vice versa happens.
In the Buu Saga, Chi-Chi is at Kami's lookout and she's struck with grief because of Gohan dying (her anger is justified though, because Gohan is her child and she only feels like how any decent mother would). She wants to give Super Buu a piece of her mind. You'd Expect: For Chi-Chi to listen to the others and not try to tell Super Buu off because she has no chance in even harming him. Instead: She walks up to Buu and slaps him in the face for supposedly killing Gohan, then Buu kills her by turning her into an egg. Then again, this is Chi-Chi we're talking about, but even she should know better.
In the Buu Saga, after Gohan and Gotenks have been absorbed, everyone starts making a big deal about how the only being strong enough to defeat Buu is Vegeto, who comes unfused after being absorbed by Buu. You'd Expect: Goku and Vegeta, both of whom had prior experience with Frieza and Cell, to take into account that the fate of the entire universe and the lives of everyone in it, including those of their loved ones, were at stake, re-fuse at first opportunity, and do everything in their power to defeat Buu. Instead: Vegeta crushes his earring, because defeating Buu using the fusion would hurt his pride. Then: Given another set of Potarra earrings, Goku immediately crushes his because he regards it as "cheating"—mere episodes after calling Vegeta out for doing the exact same thing!
In the episode "Deadly Vision," Goku and Vegeta are traveling in Buu's body and arguing. Buu pops up in his own head. You'd Expect: For Goku to shut up and try to take on Buu. Instead: He deliberately tells Vegeta that he is unable to fuse with him, right in front of Super Buu. Then: Vegeta calls out Goku on it and tells him "Nice work, is there anything else you'd like to tell him?"
Kid Buu was going to destroy the Earth with a ultra-powerful attack to made Goku and Vegeta flee for their lives. You'd Expect: Goku to grab Gohan, Goten, Trunks, and Piccolo instead, knowing that Dende and Mr. Satan would be brought back by the Namekian Dragon Balls. Regardless if he were to grab Dende or not, it wouldn't have done him much good since the Earth will be destroyed anyways, with the Balls. Instead: He grabs Dende and Mr. Satan. Goku, it's alright to save Dende, but why save the guy who stole the credit that your son deserved of beating Cell!
Kibito Kai goes to Goku and Vegeta to teleport them to safety. You'd Expect: Vegeta to grab the others, or at least Trunks and make a run for it. Instead: He blindly lets Kibito Kai teleport him and Goku to his planet, allowing Trunks, Goten, Piccolo, and Gohan to die with the planet. Worst of all, he ends up blaming Goku for the whole thing.
We note that this one is exclusive to anime filler, movies, and GT, as it is notably averted in the manga canon. Powerful warriors both good and evil come through the pike in the Other World once they're destroyed. You'd expect: Yemma-Daio and his people to dispose of the warriors' bodies before sending their spirits to their "final" destination unless keeping any of them around is absolutely necessary. (This is precisely the case in the manga.) At the very least, make sure only the good guys get to keep their corporeal forms. Instead: Everyone with a modicum of relevant power is later seen still with a body. Including the bad guys sent to Hell. You'd then expect: Some powerful dead face to stand guard in Hell making sure that any chaos doesn't last. Either make it one warrior who's generally antisocial or doesn't mind making the sacrifice, or a cycle of guys that get shifted in and out for one another (with Goku and Paikuhan being obvious players). Instead:Goz and Mez seem to be the only thing keeping the bad guys in line over there. The result: All it takes is for two evil scientists to get together and create an android, or for one Saike Oni to completely lose track of the soul cleansing chamber while listening to his music, and literally, all hell breaks loose.