Remember that flashback scene in The Lord of the Rings when Elrond and Isildur are right at the center of Mount Doom, and all Elrond needs to do to save Middle-earth from centuries of war is elf-kick that dipshit and his ring-coveting ass into the boiling lava of oblivion? Had he just done what everyone in the audience was screaming at him to do, the movies would have just been 10 hours of hobbits getting drunk.
Well, this kind of thing happens all the time -- one poor decision leads to an entire film series worth of conflict. For instance ...
#5. Batman Exists Because Ra's al Ghul Doesn't Screen His Applicants
Christopher Nolan's Batman series begins in a Bhutanese prison, where Liam Neeson's Ra's Al Ghul (who apparently has a key) recruits Bruce Wayne to be trained as a member of the League of Shadows. Ra's takes Bruce under his wing, engaging in an array of standard training exercises, including hallucinogenic ninja hide-and-seek and ice dueling peppered with damning accusations about one's father. He becomes a mentor, teaching Bruce that in order to take on his enemies, he must transcend living as an ordinary man and become an incorruptible symbol (evidently Bruce hears this as "drive around in a car that explodes into a motorcycle").
Sadly, like anything good, there's always a catch -- and after Bruce's training is complete and he knows all of the League of Shadows' secrets, he finds himself being asked to execute a man in a show of loyalty.
"Come on, you're embarrassing me in front of my minions. I told them you were cool."
He also is told that Ra's is determined to use Bruce to facilitate the total destruction of Gotham City, something he waited until just now to mention. This doesn't sit well with Bruce, and because he's already balls-deep in the organization, he ends up having to explosion-stab his way out of the League of Shadows, destroying their headquarters in the process.
The best way to show your commitment to not killing your enemies is by blowing them up.
But hey -- free training, right? And now Bruce Wayne gets to be Batman, all thanks to Ra's' total inability to make sure Bruce is cool with murdering people before training him how to bleed out of the shadows and beat the screaming Jesus out of everything on the planet.
Seriously, Ra's -- it's called a job interview, and even the freaking Olive Garden does it. They're not going to start training someone before asking if they're cool with serving soup, salad and bread sticks. Ra's is planning to kill not only an entire city, but the city that Bruce Wayne was born and raised in and where everyone he cares about still lives. Why would he just assume that Bruce would be totally fine with it? This couldn't have been mentioned while they were making Bruce stutter-step on logs while beating him with sticks?
Apparently the League's motto was "Create unstoppable ninjas first, ask questions later."
#4. Jason Bourne Loses His Memory Because He Sucks at Planning
The entire Bourne series is all about Jason Bourne trying to remember his secret agent past. Rather than take any chances, he decides to play it safe and just punch every uniform he sees, whether they be out to kill him or not.
For example, these two were just Maytag repairmen.
It's why his former employers try to assassinate him, and why they end up having to shut down their brainwashing-dudes-to-jump-from-roofs-and-ride-mopeds-in-foreign-countries program -- Jason Bourne can't remember shit, and he is going to make everything around him pay until he does. It takes him three movies and a shitload of dead bodies to figure it out.
"If you don't want the magazine subscription, just say so."
So what is the source of his amnesia, which seems to have replaced every lost memory with assbeat coupons to be redeemed immediately? Well, as the first film reveals via flashbacks, Bourne had been assigned to kill an exiled African dictator named Wombosi (you can tell how African he is by his name). Bourne used an alias to infiltrate the dictator's yacht and hide out for several days, waiting for the perfect time to strike. Everything went according to plan, except that when it was time to pull the trigger, Bourne was surprised by the presence of Wombosi's kids, causing him to suddenly grow a conscience and abort at the last minute (because as we all know, only good people have children).
It's during this hasty change of heart that Bourne gets shot in the back and falls overboard, left to float in the middle of the ocean. This causes his amnesia, because he was shot in the part of the kidneys where memories are stored.
In reality, the most common symptom of this particular injury is shark attack.
So you can trace the events of every movie back to the moment Bourne discovers the kids ... and he was somehow surprised by them? How could he possibly not have known they were there? He planned the job out to the minute, then stowed away on the yacht for five goddamned days. Did he think all the Snack Packs and Hannah Montana CDs belonged to Wombosi and his bodyguards? For a globetrotting CIA-programmed murderbot, this seems like a massive oversight. Especially when you consider what the CIA boss man tells Bourne about the mission later in the film:
You put together a meeting with Wombosi. You found the security company. You broke into the office. You're the one who picked the yacht as the strike point. You picked the boat. You picked the day. You tracked the crew, the food, the fuel. You told us where, you told us when. You hid on that boat five days. You were in, Jason. You were in. It was over.
"Wow, I'm kind of an asshole."
So Bourne put this whole thing together down to every last detail, and just completely spaced on the kids. "Whoops, my bad, guys, I just totally forgot to check." As a result, he is completely thrown off and ends up shutting down the mission, all because it never occurred to him that a militant dictator might have working sperm.
#3. Back to the Future Part II & III Only Happen Because Doc Brown Can't Hide a Body
At the end of the first Back to the Future (and consequently the beginning of the second), Doc Brown comes hauling ass through Marty's neighborhood in broad daylight, insisting that Marty and his girlfriend Jennifer join him on another time-traveling adventure.
"Great Scott! I'm abducting two minors!"
After tearing through space-time and arriving in the year 2015, Doc is forced to knock Jennifer out with a gadget because she starts asking way too many questions about the future. He babbles out a barely coherent explanation about such questions potentially causing some sort of time paradox thing ... it makes about as much sense as it needs to.
"I just wanted her to shut the fuck up."
With Jennifer out cold, Doc lands the DeLorean in a nearby alleyway and briefs Marty on his plan: Marty is to impersonate his own son and keep him from committing a crime, while Doc intercepts the real son and keeps him from ruining his life. It's bulletproof.
For this plan to work, Doc finds it necessary to plop Jennifer's unconscious body in the alley (simply leaving her in the car would have made entirely too much sense). Marty briefly protests the idea of abandoning his girlfriend next to a row of trashcans, but Doc reassures him that they will only be gone for a couple of minutes.
"Trust me, it takes at least 10 minutes to rape someone, we'll be back way before then."
Unfortunately, the future police find Jennifer almost immediately, and after a fingerprint scan they bring her to her future self's home, potentially causing that whole paradox thing Doc Brown science-clubbed her for earlier. Doc and Marty now have to rescue Jennifer, and in doing so accidentally allow Biff to steal the time machine and change the past with the sports almanac. This of course leads to the entirety of the film's plotline, which ends with Doc getting lightninged back to 1885, thus setting the stage for the third movie.
In other words, Jennifer getting picked up in that alley was the direct cause of everything else that happens in the series.
"I tell ya, Marty ... women: Can't live with 'em, can't leave 'em unconscious in a future alley."
That kind of makes you wonder why they needed to dump her body in that alleyway in the first place, considering there were a hundred ways that could go horribly wrong. Doc didn't need the extra space in the DeLorean for groceries or anything -- he was just going to use his knockout wand on Marty Jr. That's it. Why the hell did he need to empty out the car to do it?
Hell, even if Doc was planning on kidnapping Marty's son, we already know he can fit three people and a dog in that damn thing. There's plenty of room -- why the hell did he leave Jennifer in an alley? You'd think a guy like Doc Brown would know how to properly hide a teenage girl's body.