If all movies followed the same physical laws as the real world, they wouldn't be a lot of fun -- we don't want action heroes to break their legs every time they jump off a bridge and into the top of a speeding truck, we want them to roll over painlessly and continue with the chase scene (or, alternatively, to explode for no reason).
However, once you've established the rules of your movie universe, the only thing we ask is that you follow those rules. Don't say that your time traveling robot unicorn is powered by ice cream in one scene and blowjobs in the next. That's just sloppy writing.
And yet, you'd be surprised at how many classic movies break their own rules when it's convenient to the writers, like in ...
6 Inglourious Basterds -- Nobody Recognizes the Extremely Famous German Soldier
What the Movie Tells Us:
"Everyone in the German army has heard of Hugo Stiglitz."
Inglourious Basterds is about a World War II Nazi-killing squad composed of several Jewish-American soldiers and one incredibly famous German, Hugo Stiglitz. Why is he famous? Because as a soldier in the German army he murdered 13 Gestapo officers and was caught. Eventually he broke out of jail to join the Basterds.
The German media was scandalized by his Lebron-like betrayal.
Stiglitz's celebrity has spread to the point where even a lowly soldier posted in France immediately recognizes him, stating that "everyone in the German army" knows him.
"Hey, who's that guy with Hugo?"
So, it's established that Stiglitz is well known in Germany and that the Nazis know he's running with the Basterds now. He isn't just a notorious serial killer; he's a serial killer who's still out there, killing officers, so it would be in the best interest of the Germans to remember his face. Especially if they're wearing a Gestapo uniform.
The Convenient Lapse in Logic:
Hugo Stiglitz is the most notorious member of the Basterds, so what do they do? They choose him for an undercover mission that absolutely depends on him not being recognized ... and he isn't.
The Basterds are recruited for a mission to infiltrate a film premiere attended by several high-ranking German officers, and Stiglitz is among those chosen to impersonate German officials, since he actually was one. Even if his face wasn't in the newspapers (it was), that's still a dumb plan, because all it takes is one person at the premiere who served alongside Stiglitz, was present during his incarceration or simply witnessed him scalping a fellow officer to ruin the entire carefully orchestrated plan.
It doesn't help that his name and theme song tend to magically appear anywhere he goes.
The dumbest part, however, is that the plan actually works (for a while). Before they can get to the premiere, some of the Basterds meet with a spy in a bar that happens to be full of German soldiers. So Hugo "The Whole German Army Has Heard of Him" Stiglitz is face to face with other soldiers, and not one of them, not even the Gestapo officer, so much as looks at him funny.
He started parting his hair on the other side. Threw the whole Gestapo off their game.
Eventually the Basterds are found out -- not because of Stiglitz, but simply because another of the undercover agents raised the wrong fingers when asking for beers. The Gestapo officer was so observant that he immediately picked up on the accent of another person talking across the room, but at no point did he recognize the well known Gestapo killer sitting literally next to him.
"Cheer up, friend! You look like you want to shoot me."
A shootout ensues, and everyone dies except the spy, which allows the remaining Basterds to continue the plan and kill Hitler -- when, in fact, they should have all been gunned down as soon as Stiglitz walked into the room.
5 Back to the Future Part II -- Doc and Marty Can't Go Back to the Future ... But Biff Can
What the Movie Tells Us:
Once you're in an alternate timeline, you can't go back to the original one.
In Back to the Future Part II, Doc Brown takes Marty and his girlfriend to the year 2015 to save their retarded son from going to jail (instead of just telling them, "Don't raise your kid into a moron," since, you know, it hasn't happened yet). While they're doing that, dickwad Biff Tannen, now an 80-year-old man, steals the time machine and goes back to 1955 to give his younger dickwad self a sports almanac that will allow him to become rich by betting on sporting events.
Old Biff then goes to the future and leaves the time machine where he found it. Marty and Doc then head back to their present of 1985 ... only to find themselves in a 1985 where Biff is a millionaire and has reshaped Hill Valley in his ugly image.
"Welcome to the Biff Tannen Hotel, Casino and Oil Refinery!"
Doc explains that they're now in an alternate reality created when future Biff changed the past: In this timeline, Doc Brown is crazy, Marty's dad is dead and his mom is a tramp. When Marty says they should go back to the future and stop old Biff from stealing the time machine in the first place, Doc says that's impossible, because traveling from this 1985 would lead to an alternate 2015 where Biff is even richer and Marty's mom is even trashier, presumably.
"This line represents how much of a whore your mom is, Marty. She is a huge whore."
The only way to stop Biff is by going back before the timelines split -- that is, to Back to the Future I, because that was a way better movie anyway.
The Convenient Lapse in Logic:
So, according to Doc, once you're in another timeline, you can't go back to the original one ... except that's exactly what the future Biff did. The minute he changed the past, he created the alternate reality, so when he traveled back to 2015, he should have ended up in the alternate future Doc just mentioned. In fact, if he didn't get to enjoy the future where he's rich, what was the point of changing the past?
Instead, for some reason he ends up in the unchanged future ... the one Doc and Marty should have been left stranded in without a time machine.
Robbing us of a scene where Doc has to jury-rig a hoverboard to go 88 mph.
This isn't the only time Back to the Future II breaks its own logic: It's been established by now that the only way the DeLorean can travel in time is by reaching 88 miles per hour -- but at the end of the movie, when lightning strikes Doc and throws him back in time, the car was just floating there, going at no miles per hour.
Uh, uh ... wind shears ... Coriolis effect ...gigawatts ...