Since we're on the subject of Ben10, let's talk about how the Omnitrix didn't make him change into an unwanted alien in the first 2 seasons of Alien Force. It seems to be justified in season 3 of AF simply by having been screwed up at the start of the season, but this wasn't a problem in the original series. But then I realized when Ben tried using Humongosaur in one episode and he ended up being turned into Lodestar: Ben habitually uses stronger aliens. And when his ego decides to be a problem, he doesn't give much thought of strategy. The Omnitrix most likely is trying to make sure Ben's ego doesn't get in his way. —Master Knight
Word of God hinted (never outright stated) that there was an artificial intelligence of sorts in the Omnitrix that kinda chose for Ben an alien that would be better suited for the job. —Tropers/wildste
In the first series, Vilgax almost never tried to touch the watch on Ben's wrist, only trying to touch it to remove it from his wrist. Why doesn't he try to grab the watch more often? He probably knows about the watch's ability to collect DNA, and won't touch the Omnitrix unless it's off of Ben's wrist, because he does not want Ben to be as powerful or dangerous as him. Ben even lampshades in "Benwolf" where he says he could have gotten a "Wicked Vilgax".
The revelation in the finale that the previous Omnitrix was a prototype readily explains why Ben had so much trouble handling it in the original series and Alien Force. In real life, prototypes usually are pretty buggy.
But this also leads to Fridge Logic, seeing how the new Omnitrix is EVEN MORE buggy than the original.
But it makes sense given that Ben has no practice with the new interface.
People accuse Ben of Aesop Amnesia for never learning to use the Omnitrix responsibly, but take a closer look. In the first seven episodes, Ben transforms for selfish/frivolous reasons literally Once an Episode. (Wildmutt in episode 1, Grey Matter in episode 2, Fourarms in episode 3, Ghostfreak in episodes 4 and 5, Stinkfly and Fourarms in episode 6, and Ghostfreak and Stinkfly in episode 7.) Episode 7 is "Kevin 11", in which Ben is treated to the harshest Aesop regarding selfishness and responsibility that he ever gets in the show. After that, with the admitted exception of a series of Season 3 episodes, Ben's selfish/frivolous transformations become quite a bit more spaced out (check it out for yourselves). Ben really does remember his lessons... sometimes.
The mutated Kevin's powers are only 1/10 as as strong as the orignals due to being half Osmosian. Osmosians can only obtain 1/10 of an alien creature's strength when absorbing their power. This is similar to the way living things consume food. They can only absorb 1/10 of the energy food provides.
Zombozo is so callous about how he does what he does - draining the positive energies from people and Gwen looked like she was dying from it - two things: 1. This scene implies that this isn't the first time Zombozo's done this. 2. The subtext in the scene... know how Slade had almost Pedophilic moments? Zombozo has these moments, and they are even worse.
The fate of Max's partner, Phil, at the end of the second season's debut episode, "Secrets". He winds up trapped in the null-void dimension, which he had been using earlier to set up crimes by releasing aliens. When Gwen asks what'll happen to him, Grandpa replies, "There are some things about this job that are better left unheard of." Keep in mind the Null-void was designed to hold Extraterrestrial threats, many of which we later see in "Back With A Vengeance" are big and carnivorous, and Phil's just a human.
Hex is called an abusive uncle to Charmcaster due to his attitude toward her, but there's a little more to that. When Charmcaster is introduced, she makes a snarky comment to Hex, who responds "You will show me respect, my dear niece, or you will suffer the consequences." Immediately, Charmcaster becomes submissive, then takes out Hex's staff, saying she can't get it to work but swears she did nothing to break it. Hex takes the staff, explains it only works in the hands of a master magician, then shoots a magic blast out of it that knocks out two prison guards rushing to the doorway behind Charmcaster. But when the blast is fired, Charmcaster immediately and instinctively shields herself with her arms, obviously thinking it's meant for her and that this isn't an uncommon thing for Hex to do to her. Thus, Hex's absue isn't just emotional, and that explains alot more about why Charmcaster became so nasty herself.
The final battle in Destroy All Aliens between Waybig, the Retaliator, and Azmuth transformed into a berserk Waybig. It takes place in the middle of a fully populated city and racks up lots of collateral damage. It's honestly a pretty awesome fight until you realize that they must have killed hundreds or thousands of people while fighting...
Zombozo stole from a brain bank. Consider what zombies do.
Ectonurites are capable of retaining their consciousness from a single strand of DNA. They could theoretically create a civilization-sized army from their greatest generals.
At the climax of Don't Drink the Water, Gwen tricks Toddler Heatblast by constantly teasing him into having a meltdown (and thereby evaporating the Fountain of Youth of which Hex is using to power himself up.) It works, but if you take science into mind: What will happen when that evaporated magic water condensates and turns into rain? Nice job Gwen...