The Cullens are playing baseball, when they're approached by three vampires, Laurent, James and Victoria, who become hostile upon noticing the human Bella there. Since Bella happens to be Edward Cullen's girlfriend, he has no intention of allowing the unfriendly vampires to have her, and neither do his family. Note that all seven of the Cullens are present here. You'd Expect: The Cullens to stand their ground and warn Laurent's group to get lost, if they don't want them to just kill them. It's not like they have any reason to feel threatened, given that Laurent's group is outnumbered more than two to one. Instead: Edward insists on getting Bella out of there immediately, and the Cullens flee. This results in Bella having to break her father's heart, Bella nearly dying at James' hands (see the below point) and the issue just taking far longer to get resolved than it needed to be.
James has tried attacking Bella and Edward wants to get her out of Forks for her own safety. The evil vampires call Bella and tell her they have her mother hostage. You'd Expect: Bella to tell the Cullens about this; Edward and his family are nigh indestructible, much more knowledgeable about vampires in general, have superpowers, and can just downright handle it much better than she can. Instead: Bella goes off on her own and is nearly killed.
Edward leaves Bella for her own safety. After some months, he hears a rumour Bella died. You'd Think: In an age where every teenager on the planet has Facebook, Twitter, e-mail, or a cell phone, or at least knows what a pay phone is, that he could check extensively to see if this was true. Instead: He does do a bit of checking, in the form of making a single phone call to her house. Someone who doesn't live there answers the phone, and when Edward asks for Charlie, Jacob ambiguously says that Charlie is organizing a funeral. And like a Three's Company character, Edward assumes the worst and reacts in the most asinine manner he can come up with: suicide.
After having a vision of Bella jumping off a cliff, Alice goes down to Forks to see if Charlie is okay, and discovers that Bella is in fact alive. You'd Expect: Alice would call home and let the other Cullens know about this. Instead: Alice stays with Bella for two days without calling home once, so they still believe Bella is dead, and Rosalie informs Edward of the fact, leading to the mess described above.
Jacob forcibly kisses Bella at one point. Her dad Charlie later finds out about this. You'd Expect: Charlie to be furious at Jacob for forcing himself onto his daughter. Instead: He approves of Jacob's actions. Sure, he doesn't like the idea of Edward and Bella being together, but he can't be that desperate for her to be with someone else without being either an idiot or a jerkass.
After the battle with the newborn army, one last newborn shows up and tries to attack the Cullens and werewolves. Leah engages him, and Jacob decides to join in soon afterwards. You'd Expect: Him to offer to help Leah with the newborn, or just take him down while he's distracted by Leah. Instead: He decides to just shove Leah out of the way, apparently under the impression that Leah can't handle this on her own, despite her being perfectly capable of defeating newborn vampires in single combat. As a result, the newborn is able to badly injure him while he's busy with Leah. Not Only That: Everyone blames Leah for Jacob getting hurt, never mind that it all happened due to Jacob's ineptitude.
The Short Second Life of Bree Tanner:
The titular character is able to recall her memories of being turned into a vampire. This involves being violently kidnapped and hearing specifically that she will be used as a "distraction" for something. She and her friend/love interest have at least two discussions about how they suspect something weird is going on and that they're being used as pawns. They find out that they were lied to about what rules vampires follow to survive. They know that their leader, Riley, can't be bothered to keep them safe from one another and actively goes out and kidnaps other teenagers. You'd Think: They'd run the hell away as soon as they figured out that sunlight doesn't kill them. Instead: They spend most of the story pondering what's going on. When they finally decide on a course of action, the love interest decides to confront Riley. Alone, at dawn, in an unknown location. Bree spends most of the rest of the book wondering where he could possibly be and what's going on, instead of running away or at least sneaking off to look for him.
Early on, Jacob disappears from Forks, as a result of Bella choosing Edward and not him. Charlie, who likes Jacob, and happens to be the Chief of Police in Forks, doesn't know why he's gone, and is understandably worried. Jacob's dad, Billy, assures him that his son is perfectly fine on his own. You'd Expect: Charlie to trust Billy, and just leave it at that. Or if he's still worried for Jacob, he could try and get his opposite number in La Push to do something about him. Instead: Charlie tries to organise a mass manhunt for Jacob using Forks Police resources, despite Billy's assurances that Jacob's perfectly fine, and the fact that Jacob doesn't even live in Forks. In case you're unaware, this last point makes Charlie's actions illegal.
While Bella and her daughter Renesmee are out in the snow, they are spotted by the vampire Irina, who suspects that Renesmee is an immortal child (a vampire who was turned at a very early age) created by the Cullens. You'd Expect: Irina - who possesses great eyesight, like all vampires do - to notice that Renesmee has brown eyes (vampires either have red or golden eyes), doesn't sparkle in the sun (as is the norm for vamps in this series) and is clearly hunting animals (no-one has been able to teach an immortal child to do this). Those are the most obvious ones. From this, she could deduce that Renesmee isn't an immortal child. Instead: She doesn't, and since immortal children are a big no-no in vampire society, she goes and reports Renesmee to the Volturi. The mess is sorted out before any of the vampires can come to blows, but not before Irina is killed for her mistake.
The Cullens aren't any better when it comes to dealing with the Renesmee issue. The way things are going at this point in the book, the Volturi will come and kill them all. You'd Expect: The Cullens to just get in contact with the Volturi and explain things. The head of the Volturi happens to be on good terms with Carlisle, has let the Cullens off for previous offences (such as letting human Bella associate with vampires), and is also a real stickler for the rules, apparently. If the Volturi want proof that Renesmee isn't a full vampire, the Cullens have a scrapbook that clearly shows her physically aging. That, and the Volturi head can view all of a person's memories, which would allow him to see Renesmee growing up. Instead: The Cullens at first assume that they're doomed either way, and later waste time gathering a load of other vampires to testify that Renesmee isn't what Irina thinks she is. Not only do the Cullens ignore the obvious solution to their problem YET AGAIN, a number of humans are killed by the vampires the Cullens invite over.
And finally, there's the Volturi's actions during the Renesmee issue. You'd Expect: Aro to first confirm that they're really dealing with an immortal child here, either by scanning Irina's memories, or by having one of his many subordinates pay the Cullens a visit. Instead: He buys it right away, amasses an army of vampires to deal with the Cullens, and leaves humiliated once he learns the truth. Speaking of dealing with the Cullens... You'd Expect: That considering they're apparently dealing with an immortal child here, the Volturi would take quick and immediate action to end the threat. Hell, they could just send Alec and Jane, who had once curbstomped an army of 100 vampires, and it would be enough. Instead: They spend a month gathering witnesses before confronting the Cullens. Considering that one of the reasons immortal children are banned is due to them being able to completely wipe out human villages...not a smart move, Team Volturi.