Cold Chicken

So you know how in the first Harry Potter book, all problems could have been resolved at any point if the kids had just gone to Dumbledore? And you know how in the second Harry Potter book, all problems could have been resolved at any point if the kids had just gone to Dumbledore? And you know how in the third Harry Potter book... you get the idea.
That realization is an example of what Alfred Hitchcock once called an icebox scene - a plot element that makes no sense, only you don't realize it until you're at home pulling cold chicken out of the icebox. 
Some of my favorite icebox moments.

•  In The Avengers, Thor's hammer has destroyed everything he has ever hit with it. Meaning that when he, sparring with Iron Man and Captain America, had no idea he wasn't, in fact, going to murder them.
• In The Avengers, where's Loki while everyone's sitting around eating shwarma?
• In Harry Potter, if Peter Pettigrew showed up with his own name on the Marauder's Map, even when he was in the form of Scabbers, Ron's pet rat. If the Weasley twins had been in possession of the Marauder's Map up until they gave it to Harry, then they would likely have noticed their brother Ron sharing a bed with a strange man every night.
• On CSI, pretty much none of the technology they use to catch bad guys exists. You probably figured out that ATM cameras can't show a mole on the elbow of a person a mile away. But there's stuff even more basic. Anyone who owns a cat knows that hairs get tracked everywhere all the time, and finding a single hair at a crime scene doesn't mean jack. I mean, if I make my mom a scarf at my best friend's house in DC, and my best friend's dog sheds in it (because that's what her dog does for a living), and my mom gives the scarf to a homeless guy because she's deathly allergic to dogs and thus the scarf, then there's a chance there's a homeless dude wandering the streets of Cleveland with hairs on him from a dog in DC. That doesn't mean he murdered the dog. (Don't ask yourself why my mom didn't just wash the scarf - that's another icebox moment). So anyway, in the real world, there are going to be dozens, maybe hundreds of hairs floating around any given crime scene, and DNA testing is expensive. Even if you tested every piece of hair around the crime scene, the chances of matching it to the bad guy and then using it to hang him are... somewhat less good than TV would have us think.
• How the hell does Marty McFly's mom not notice that her son looks exactly like that dude she hit with her car and tried to sleep with when she was in high school?
• The X-Men leave more bodies in their wake than probably all their enemies put together. There's even a whole super villain team made up of the few people that Wolverine has left alive. And then there's the whole Phoenix murdering five billion people thing. In fact, countless lives could have been saved if the X-Men had just let Magneto enslave humanity in 1963. Because really, how were he and half a dozen freaks with sorta lame powers going to rule the world anyway? 
• Movie and TV characters always have way more money than they have any business having, unless it's a plot point that they don't. Like where did Juno get the money for all those pregnancy tests, especially from a convenience store? Those things - especially the fancy kind that don't make you pee in a cup - cost like $15 at WalMart - and would cost a lot more at a convenience store. And how many did she say she'd taken? Half a dozen? That's a huge chunk of change to a teenager, especially one from a not so rich family. She clearly doesn't have a job... 
• In Dogma... there's a little flaw in Bartelby and Loki's plan to get back to heaven. According to a Catholic tradition, anyone who walks through a church on its centennial is forgiven of all sins. Bartleby and Loki are going to thwart God's will by going through the doors to a church on its centennial - however, in order to truly be absolved of one's sins, according to the church, you must genuinely repent and intend to sin no more. If you're walking through the doors with the intention of thwarting God's will, then you're sinning. If you can get around that, since the cops were sure to shoot them when they left the church, then leaving the church would be suicide, a mortal sin.