Episode "Back With a Vengeance", Ben and Gwen are trying to escape the Null Void, a horrific pocket dimension. However Ben's arch-nemesis Kevin is trying to escape before them. Then Ben calls to him, saying "I'm over here Kevin! Come get me!" You'd Expect: Kevin to do, as he said a few minutes ago, "leave him stranded in the dimension with his arch nemesis, which is a satisfying enough revenge. Instead: He charges at Ben to kill him. A head hop and a few jumps later, Ben is home free. Kevin is just plain screwed.
In the episode "The Galactic Enforcers", Ben and the titular Galactic Enforcers are fighting two crazed and deadly mercenaries called Sixsix and Volcanus. Gwen and Grandpa Max are locked in the Galactic Enforcers' ship, being not allowed by the Enforcers to be involved with the business below. You'd Expect: Gwen and Max to stay where they are. They are both unarmed humans and Max doesn't even have access to his Plumber equipment. They are not going to be able to dent the two villains in the state they are in now. Instead: Max hacks and activates the teleporters and he and Gwen teleport, Leeroy Jenkins style, down to the location. Upon arrival, they IMMEDIATELY get in trouble and Ben has to save them, allowing Sixsix and Volcanus to get away with what they wanted.
"Kevin's Big Score" - Kevin finds out an old partner of his in the blackmarket alien tech underworld has a Holoviewer containing a message for Ben from his grandfather. You'd expect: Kevin to share this information with Ben and Gwen so they could come up with a plan to buy, trade for or steal it. Instead: Kevin steals the Rust Bucket to trade it because he doesn't want to bring up his past - a past Ben and Gwen already know about! - for fear they wouldn't trust him again. But stealing their grandpa's house with no explanation is sure to gain their trust...
"Above and Beyond" - Alan comes up with an idea, but before saying what it is, believes it to be stupid. Pierce, as the auxiliary leader of the group, asks for the suggestion anyway, and it turns out to be to use the Plumber's Badge locator feature to find Grandpa Max, who is clearly trying to hide from Ben as part of the secret training for the Plumber's Helpers. You'd Expect: Pierce to realize from Alan's questioning of his own suggestion that Ben may be using the locator himself to make it easier for himself to find Max. If he is to use the locator, it would be better to do so to keep track of Ben's location and work off of that. Instead: He just goes along with the suggestion without second thought or any other sign of major caution. He gets separated from Alan and falls into a trap where he is isolated against Ben. Cue being on the rough end of a Curb-Stomp Battle as a reward for disappointing sense for a leader.
Ben 10: Ultimate Alien: Darkstar manages to find a way to Ledgerdomain, a realm made of mana that can feed his need for energy. While there, he meets Charmcaster, who rules said realm and controls its power. Despite having a somewhat insane sense of fun, she is completely smitten with him. Hell, it is all but stated outright that she slept with him. In addition to that, she's willing to give him however much energy he needs to destroy his enemies. You'd Expect: Darkstar to realize he's hit the motherload, and not do anything to betray the trust she's put in him. Instead: Complete sociopath that he is, Darkstar not only persists in acting like she owes him more, but practically ignores her entirely. He boasts about taking over Earth and her dimension and continues to lust after Gwen (who, remember, wants nothing to do with him). Predictably, she depowers him then tosses him back to Earth like garbage. For added stupidity: The above mistakes weren't actually enough to completely alienate her. All he had to do was remember her real name: Hope. Just one simple little name. He can't even do that!
The Ultimate Alien episode "Duped": Ben wants to see the new Sumo Slammers movie, while Kevin wants him to help track down the Forever Knights and Julie wants him to watch her tennis game. Taking advantage of his Echo Echo form's powers, Ben splits himself into three versions: one logical, one compassionate, one aggressive. You'd Expect: Compassionate Ben to watch Julie's game and Aggressive Ben to help Kevin fight the Forever Knights. Instead: Compassionate Ben is helping Kevin, generally creeping him out by constantly trying to talk about their feelings. Aggressive Ben is watching Julie's game, generally being an ass and ruining both players' concentration.
In Vilgax Must Croak, Ben is charged with taking his Arch-EnemyVilgax to custody to Incarceon: You'd Expect: Considering the last time Ben fought him had Vilgax Scam Diagon, he would act serious and concentrated during the mission, constantly keep an eye on him and be ready for anything. Instead: He treats it casually, consantly boasts about how he has defeated Vilgax a thousands time, and even at some point allows Vilgax to get temporarly free to help. Needless to say, by the end of the episode, Vilgax has not only escaped, but actually had Ben at his mercy, and our hero only survived because the overlord ended up making the same mistake than Psychobos.
In Rule of Engagement, Ben is playing video games when his girlfriend Julie calls him: You'd Expect:: Ben to stop playing just the time to answer the call. Instead: He keeps playing while answering, resulting in his frustration and insult at the game being interpreted by Julie as directed toward her, and she breaks up with him. For added stupidity: He didn't call her back when he noticed she wasn't calling anymore. By the time he meets her again, she already has moved on and found a new boyfriend.
In Special Delivery: Ben, is forced to acccomplish delivery for his Neightboor Mr Beaumann as a payment for destroying his car. During the delivery, he accidentally ends up exchanging a goldfish he is supposed to deliver with a Dwarf Star, an artifact of great power thug leader Psyphon was about sell, causing everyone in Undertown to attack him so they can get it. When Psyphon finally confront him about it, Ben, having no idea what the Dwarf Star is, offers to give it back if they just give him back the goldfish in exchange and let him finish his delivery in peace: You'd Expect: Psyphon to just shut up and accept the offer, so everyone can go his way without further trouble. Instead: The idea Ben would agree to trade something so powerful for something so worthless is so confusing to him he feels the need to voice the thought, including what the Dwarf Star really is. Needless to say, Ben immediatly removes his offer and proceeds to swoop the floor with his minions.
Similarly, after the avobe events happened, Psyphon finds himself alone with the Dwarf Star while Ben is busy kicking his minions' ass. You'd Expect: Psyphon to just leave while Ben isn't seeing him. Instead: He uses the Dwarf Star on himself and attacks Ben. After a fairly difficult fight, Ben wins and takes the Dwarf Star back.
In Mud Is Thicker Than Water, the Plumbers knows they have a mole inside their headquarters who is stealing their tech and selling it to criminals. At the same time, they ends up having to deal with the Anihilarg, a Doomsday Device capable of destroying the entire universe that was originally broken, but that their scientists Blucik and Driba somehow fixed: You'd Expect: The Plumbers to just destroy the Anihilarg again, and this time dispose of the remains so nobody will be able to fix it. After all, there really is no point for an organisation in charge of protecting the galaxy in keeping a device that can destroy the Universe, and its previous destruction prove you can dispose of it easily and without backfire. Instead: They decide to move it to another planet. The escort ends up attacked by Psyphon's thugs, and the mole steals the Anihilarg in the confusion.
In Return to Forever, Ben sees Forever Knight member Sir Morton kidnapping his friend and fan Jimmy Jones: You'd Expect: Ben, being a shapeshifter with no less than three forms having Super Speed as their main power, and even more capable of flight, to just turn into one of those and catch up with Morton; Instead: He tries to chase Morton as a human, and settle on using his friend's vehicule when that doesn't work.
In Animo Crackers, Dr. Animo's future self travels back to the present to help himself escape Plumber HQ and defeat Ben. You'd Expect: The two Animos to work well together and be a threat. Instead: Future Animo explains Ben's new alien's powers for him. The two Animos then end up fighting each other, allowing them to easily be captured.