The first What an Idiot moment should be when Ranma and Genma accidentally fall into the Jusenkyo spring and turn into (respectively) a woman and a panda when they are in training. You'd Expect: That they would talk to the Jusenkyo guide and see if he could help them sort the curses out. Instead: They decide to head to Japan. And they never even consider finding the cursed male spring so that they can turn back again. But really, if they did, will Ranma ever be born to entertain us? Also: This applies to almost every Jusenkyo victim. They all fell in the springs, and never thought about reversing the curse until they were an entire country away, instead lamenting their cursed nature and swearing revenge on those responsible.
Genma and Soun have agreed to have their kids (Ranma, and one of the Tendo girls) marry each other, and unite their martial arts schools. You'd Expect: The pair of them to not let the kids know about this plan, and subtely work on making them get along. Instead: They flat out tell Ranma and Akane that they are to marry one another almost as soon as they meet. This immediately causes friction between the two, which the fathers make no attempt to smoothen, and makes them reluctant to get closer together for most of the series. You'd Also Expect: Genma not to make any other deals that involve marrying Ranma to anyone else while they're on the road. Instead: He does, and these generally end up coming back to bite him in the ass in the form of another fiancee for Ranma, making it even harder for Genma and Soun to marry him off to Akane.
Ranma is a boy who is cursed to transform into a girl every time he comes into contact with cold water & vice versa with hot water. He is uber-masculine and absolutely hates having his life ruined by these transformations. You'd Expect: Every single time he leaves his house he would carry a backpack containing at least a Thermos of hot water, a kettle, a full body anorak and a towel. Instead: He leaves his house every morning wearing a short sleeveless vest and without precautions of any kind. This frequently turns out exactly how you would expect (not helped by Japan's rainy climate).
Akane has been given an unbeatable powerup thanks to a Battle Dougi, Soun and Genma convince Ranma to seduce her, since this would be one way to make the Dougi desert its master. Ranma does, but quickly realizes he's being a Jerkass and stops the charade, allowing his true feelings to surface instead. They're on the verge of kissing... You'd Expect: That Soun, Genma, and Nabiki would back off, since they actually want Ranma and Akane to get together conclusively, and allow the scene to reach its expected outcome. Instead: Nabiki gleefully lets the Dougi interrupt the moment, and Soun reveals the entire "seduce the Dougi's master" plot to Akane. She does not take it well.
Ranma, who has a surplus of fiancées, suddenly has Shampoo, a fiancée he's not interested in, to lose interest in him. (She's been brainwashed by a magical jewel that alters her behavior, but Ranma doesn't know that.) You'd Expect: That Ranma thanks the gods that something has finally gone right in his life and he is now a significant step closer to being free of his tangled web of romance. Instead: He tries to win Shampoo back, not because he's worried about her being brainwashed, but because his ego can't handle being rejected by a woman. Any issues he has involving Shampoo from then on are by definition self inflicted. Also to note, he never learns she was brainwashed, not even when Akane, now brainwashed herself, breaks things up. Not to mention, Brainwashed!Shampoo takes his most forceful try as pretty much a rape attempt, and it's among the few times she looks genuinely better than Ranma; even with her brainwashing, she does have a point about how you can't force people to love you.
Nabiki has been given the "rights to Ranma" after Akane throws a temper tantrum. She knows that Ranma's Unwanted Harem consists of an Amazon whose culture has a motto of killing obstacles, a Rich Bitch who routinely uses blackmail, paralysis powder and other dirty tricks on Ranma himself and is much nastier to her competition, and a Wholesome Crossdresser who hunted Ranma for about ten years looking for revenge. They also know that Ranma doesn't have the "history" with Nabiki that he does with Akane, so they're more likely to believe they can get away with hurting her. You'd expect: She would keep things on the quiet and make nice with Ranma, who is her only protection against them should they think to try anything. Instead: She flaunts her "new relationship" in front of everyone at school, treats Ranma like crap, and then goes so far as to offer to auction him off to all of the girls simultaneously... naturally, they egg each other on and try to kill her instead.
Nabiki has learned that Ranma and her sister are finally going to get married. You'd expect: She'd try and keep the various also-rans away from the ceremony, if only to prevent either her own death or the destruction of her property in the crossfire, being well aware that the mere possibility that Ranma was trying to give Akane an engagement ring caused the girls to destroy the Saotome house. Instead: She figures they will bring cash presents to the wedding, as is traditional, and that she can somehow take that money for herself (as well as any money she may get from telling them). So, she calls them and lets them know what's going on.
In an anime episode, Shampoo has managed to use a literal Red String of Fate to get Ranma to fall head-over-heels in love with her, and the effect will become permanent if they are still bound together five hours after it was first used to tie them together. You'd expect: She and Ranma would high-tail it out of town so that nobody can find them until the magic is permanent. Instead: She not only sits around the Tendo Dojo, giving Kasumi and Nabiki a chance to figure out that the string is responsible, she sends a wedding invitation to Akane. And then she trusts Akane enough to let her get close enough to cut the string, seconds before the magic expires and the string vanishes — releasing Ranma back to his senses.
Ranma and Akane's persistent and unwanted admirer Kuno has met Mariko Konjou, a girl with a similar personality to his own and who is actually very interested in him. You'd expect: Ranma decides to do some Cleaning Up Romantic Loose Ends and encourages Kuno to give up on his female form and Akane to get together with his admirer. Instead: Ranma decides to break them up by proving he (in his female form) loves Kuno more and actively encourages Kuno to come after "her" and dump his new love interest. All to get back at Kuno's admirer for making Akane upset by cheating in a volleyball game, never mind that there were other ways to get back at her. Ultimately, Kuno's admirer gives up and leaves, while Kuno continues to pursue Ranma and Akane as much as before if not more so and ends up ruining Ranma's wedding in the final story.