When Mitsuko is caught trying to kill Tadakatsu, he decides to hold her captive for a while and make sure she doesn't try and kill him. You'd Expect: He continues to keep an eye on her due to her terriblereputation. Instead: He not only trusts Mitsuko, but tries to screw her (despite her reputation)
The girls in the light house seem like logical, level-headed individuals at first. Once they find out someone tried to poison the food they freak out. You'd Expect: At least one of them to take charge and rationally reason who did it. Instead: Each of them proceeds to freak out and spray bullets everywhere. Bonus points in the film since once everyone is dead one of the girls asks the real murderer if she did it and the girl shakes her head no. The whole thing would've been solved if she trusted the word of her classmates like she did then.
Mitsuko has a moment in the manga when she tries to seduce Kiriyama of all people. You'd Expect: Before the game Mitsuko determined which guys seemed easy to seduce. Apparently, Kiriyama's Dull Eyes of Unhappiness and expressionless face weren't a big enough clue. Instead: She goes along with the plan and ends up failing by exposing herself out in the open.
Hirono also gets one in the film. She holds Mitsuko at gunpoint while the latter has no gun. You'd Expect: Hirono kills Mitsuko quickly since she knows how dangerous she is. Instead: She kicks her while her down and spends so much time doing so she ends up getting zapped by Mitsuko's stunner and ends up with her gun stolen.
Toshinori Oda in all versions receives a bulletproof vest, and is gunned down by Kiriyama but survives. You'd Expect: He'd play possum and wait for Kiriyama to move along (and, to be fair, he DOES do this in the novel but Kiriyama goes back for a Double Tap). Instead: In the film, after Kiriyama leaves, Toshinori actually shouts out "I survived thanks to my awesome bulletproof vest!"...at which point Kiriyama returns and beheads him with a sword. In the manga, he fakes a death rattle in the hopes that Kiriyama would approach to check on him, at which point he'd shoot Kiriyama. When Toshinori springs to life, Kiriyama quickly jumps between Toshinori's legs and fires a volley of bullets into his unguarded dick.
Mimura has a plan to hack into the computer system of the school and disable the collars, then proceed to escape from the island, which he tells to Yutaka. The plan fails when the attempt is foiled and he realizes that their collars are bugged for sound. You'd Expect: Mimura to realize that a paranoid government that made sure to explain to them that there is no escape from the island or game would also keep tabs on the students not just by the collars, but also via sound. Instead: The plan fails, they think up a more dangerous, but also more direct plan on just bombing the school and escaping that could very easily go wrong. The plan would've likely worked, but Kiriyama came across them before it could be pulled off...
Yumiko and Yukiko are two best friends and do not want to fight. Thinking that the others are just as scared as they are, proceed to head to an observation plateau and shout into a megaphone where they are, that they do not want to fight and everyone should proceed to this place to join them. You'd Expect: Those two to realize what an idiotic idea that is and either drop the plan immediately or proceed to shout out and then hide themselves, to be safe. Instead: They remain where they are and get blasted by a nice big volley of Kiriyama's sub-machine gun and their dying screams are heard through the megaphone. Also: Shogo actually fired two warning shots into the air, to try to make them realize what a terrible idea that is and have them run away to safety, which also put himself, Shuuya and Noriko in danger. And yet those two still remained where they were, shouting more into the megaphone how they do not want to fight. Also, what goes up must come down, so Shogo could have inadvertently shot someone when the bullets fell back to earth.