People often complain that Marty didn't conquer his fears or do what his dad did and get extra confidence to get his dream career. But Marty was terrified that people were going to reject his music, and only did the audition in front of a few teachers, but he got over it and managed to do it in front of hundreds of people. They didn't like the end of it, but he is so confident by the end of "Johnny B. Goode" that he says "I guess you guys aren't ready for that yet. But your kids are gonna love it!" He might not just be talking about Rock & Roll but about his music as well.
It seems inconceivable that George, no matter how rage-fuelled, was somehow able to knock out Biff in a single punch. But looking at the punch closer, George originally tries to hit Biff with his right hand, only for Biff to grab him and put him in an arm-lock, and it's only when Lorraine is pushed to the ground that George spins round and smacks Biff with his LEFT hand. Now earlier in the film we see George writing his sci-fi stories in the lunch-room, and he's using his right hand. But even back in the 50s, it was commonplace for schools to insist on right-handedness even if a person was born a lefty. In this case, it's possible that George was born left-handed, forced to pretend he was right-handed by school (since we see him writing with his right-hand) but still managed to maintain a degree of dominant strength in his left hand. As such, when he went up against Biff, of course his right hand was useless, whereas his left hand was stronger.
In Real Life, the early model DMC-12s were plagued with alternator issues. The battery was infamously unreliable when several appliances were running, leaving drivers stranded (most famously, Johnny Carson was a victim of the DMC-12's poor battery performance). The frequent stalling of the DeLorean in Part 1 could be plot convenience or it could be a reference to the Delorean's reputation for unreliability.
One may wonder why 1955 Lorraine/her family would take Marty's pants off while he was knocked out. Since the car hit Marty in his leg/shin/thigh area, they probably took his pants off in order to check for bruises or any other serious injuries. Obviously he wasn't injured, so they just forgot to put his pants back on or had a hard time doing so.
When Marty first shows up at 1955 Doc Brown's house in the first installment, he's testing a thought-reading device. He makes a series of guesses as to why Marty is there, ending with noticing Marty's jacket and asking if he's looking for donations for the coast guard.
Why does the DeLorean lack a rear-view mirror? Not only because vision the back is blocked off by the time machine engine, but to avoid reflecting the light of the flux capacitor into the eyes of the driver. (There's a label on the glass which warns of blinding light.)
If the Libyans knew anything about building a nuke, they never would have gone to Doc Brown in the first place.
Doc Brown burned down his house, wasted his family fortune, and spent 30 years trying to invent his time machine. In 1985, he is reduced to living in his former garage next to a Burger King, driving a shady truck advertising "24 HR Scientific Services," and hanging out with ignored and insecure high school kids, all the while being treated as an outcast by his community (even Strickland calls him a "real nutcase"). It's plausible that Doc has cultivated black market connections in order to make some money and get the raw materials he needs for his experiments and inventions. These connections probably helped him get in touch with the Libyans and even corrupt officials at the nuclear research center in order to get the plutonium he needed for the time machine.
Also in the first film, regarding Marty only setting a few minutes' worth of lead time in returning to the past: keeping in mind that Marty didn't want to screw up the timeline worse than it already was (he knew he couldn't run into himself in the future/present), he gave himself as little time as possible to screw things up in. Also, he wasn't expecting the DeLorean to fail on him.
In the second and third movies, Marty suddenly seems to have a problem with being called "chicken" (or "yellow", etc.) Although it seems to come out of nowhere, but in the first film no one calls him chicken, they may call him a lot of things but no one really questions his bravery.
Regarding the Ripple Effect-Proof Memory in the first movie; Marty's actions in 1955 caused his brother and sister to be completely erased, then remade, in time. However, he himself was only partly erased before being brought back. This would explain why he kept his old memories, whilst still having some personality changes (e.g.; the "chicken" problem, deciding to send his demo tape into the record company after all), even though the rest of his family are now completely different.
It's made clear that Hill Valley is kinda crummy and pretty inconsequential. But Marty and Doc are very lucky that this is the case, because it makes it the perfect testbed for time travel. No matter how much they alter Hill Valley's history, it won't greatly affect the rest of the world, which makes messing up history less catastrophic, and makes it safer to alter history for the better (e.g. Marty's family, Marty's kids and Clara Clayton). This is explicitly stated in the Telltale game, in which changes that could potentially affect the world fail to to do so miserably; in Episode 2, the prevention of Biff's gangster father's imprisonment results in the existence of the fifth biggest crime syndicate in the state, and in Episode 3, the alternate Hill Valley's strict societal model only has one other adopter, the already law-heavy Singapore.
This wasn't always the case, however. In BttF2, Biff's meddling with the timeline somehow got Richard Nixon elected to at least four terms and the Vietnam War is still going on as of 1983. Though it could be argued his meddling was financial in nature rather then social giving him a lot more room to influence those outside of Hill Valley. After all it really isn't of world-changing significance if someone did or did not get laid at the prom, but millions upon millions of dollars suddenly ending up in a different set of hands...
Here's an example from Back to the Future I never thought of until I saw it for the umpteenth time: Marty's father's explanation for why he was in the street to get run over by Lorraine's father? Birdwatching. Well, he was packing binoculars, but that's not the only reason it's appropriate. He was there looking at Lorraine. What's 50's slang for an attractive young woman? A bird. He was, so to speak, literally bird watching.
The first movie makes mention of Marty's uncle Joey who is perpetually in prison for one reason or another. Once he goes back to 1955 he sees his uncle as a baby who is always sitting in his crib and his mother (Marty's grandmother) says that he just cries whenever they take him out so they just leave him in there and move the crib around the house. It wasn't until years later that I realized Joey feels insecure without the bars and most likely suffers from some mild form of mental illness and thus commits crimes to get into prison and thus feel "safe". In the altered timeline, George has enough money to get him the help he needs.
But Uncle Joey is never mentioned in the improved timeline. It's never said if he's still in prison or not. Though a deleted scene from Part II shows that Lorraine was expecting Joey to get out of prison again in 2015 (and Marty telling her they should wait until he's been released to celebrate), while the comic book has a story arc where Joey does get out in 1986.
So in the first movie, Lorraine falls in love and later marries George after he is involved in a car accident because she feels sorry for him. In the second movie, it is implied that Jennifer marries Marty because she feels sorry for him after he is involved in a car accident. Can't believe it took me so long to realize that.
So he avoids an unhappy marriage that started on a weak foundation. Hardly that horrifying. Maybe he and Jennifer get married for better reasons?
Heck, even avoiding a stellar marriage seems uneventful, for some that have witnessed love coming and going like the breeze.
And hey, George and Lorraine's marriage was way happier in the timeline where he wasn't involved in a car accident.
If Marty was so influential to George and Lorraine that they named a kid after this guy they met, how come it's the name of their third child instead of their first? Simple, he really wasn't that influential by the time their first child was born (8 years or so after they got romantic). Whatever inspiration that made them come up with the name Marty in the original timeline (friend, celebrity, name from a baby book, etc.) is still the inspiration for them in 1967 or so when Marty is born, albeit with a little extra emphasis "Oh, that's the same name as that guy who helped us meet each other, wasn't it?"
Working this out in a vacuum... (shut up; some of us who didn't grow up with the internet don't realize that pre-internet subjects are scrutinized by anyone on the internet.) I always assumed that Doc was just crazy and had set all of his clocks wrong, not that his house being non-synchronous was an earlier experiment... until the umpteenth rerun.
And I've assumed that Marty had always been vulnerable to coward-adjectives: complete immunity to ripple effect still inconclusive.
Why does Doc Brown continue to insist not letting Marty tell him about his death? It wasn't just because he didn't want to know what's going on, but from the sequence he saw when he saw the Oh, Crap! moment on the video he watched, he figured out that he would die. And because he knew of his death, he didn't want Marty to worry about his own death.
Or that he figured that knowing the future locks you into it, making it a "fixed point."
Retroactive Fridge Brilliance: In Doctor Who, the Doctor manages to fake his death, but makes it seem as if he died, in order to keep a fixed point from being fucked up. Doc Brown probably came to a similar conclusion; as long as Marty thinks that he was dead, he would still go into the past, and time would continue on its natural course.
This one isn't all that hard to figure out. Doc was watching a video pertaining to the development of his time machine, and Marty at that moment first chooses to try to tell him about the future. From that point on Doc does not want to know anything about it. There are a lot of good reasons for why this could be. First, by his own admission the time machine was the first thing he invented that worked. Considering he spent his entire life as an inventor without success, imagine the vindication for him when he learned he not only eventually created something that works, but something as amazing as a time machine. Now imagine if he perceived a chance to never achieve that ambition if Marty told him something he did not want to hear. If Marty told him something that could make him change his future, then he could very well never invent the one thing that keeps his whole life from having been a failure. Consider also that Doc can see that Marty has quite clearly screwed up his own future. Doc by contrast does not want the same thing to happen to him. Even if he guessed the information was about his death, all not knowing would mean is that he would die when he always died. At the end he puts in point blank when he says he refuses to accept the responsibility. For all his brilliance he cannot foresee what the future will be, he was not willing to risk changing his future or that of anyone else connected to him any more than he had to. It takes him many years and an obviously great amount of curiosity before he reassembles Marty's letter and receives the warning.
Typically, a helicopter indicates to the audience that Marty is indeed back in 1985: we see one at the end of Part I (with the searchlight shining on the Clocktower) and we hear one at the end of Part III (when the DeLorean is drifting past the Eastwood Ravine sign). However, we don't hear one right away upon landing back in 1985 in Part II, helping the audience to believe that Marty may be stuck in the wrong year. However, we do hear a helicopter right at the moment when Marty gets the date from Strickland's newspaper, confirming that we ARE in 1985—just not the right one.
Technically subverted in a doubly brilliant method since when Doc, Marty, Jennifer and Einstein return from 2015 to 1985A, they hear an AEROPLANE instead of a HELICOPTER, it's also a flying vehicle but not the SAME one we'd expect, much like how they ARE in 1985 alright, but not the SAME 1985 that they know.
A double example: First, the paralleling color-themed names of the two mayors of Hill Valley in Part I, Red Thomas and Goldie Wilson. Secondly, could their last names be a reference (even unintentional) to Thomas F. "Biff Tannen" Wilson?
I couldn't ever explain why Doc thought asking who the president of the United States was would be a foolproof plan to catching Marty in his lie; after all, how would Doc know if he was lying? But what is his reaction to Marty's answer? "The actor?!" He thought that Marty, being just a kid, wouldn't be clever enough to think up a plausible name off the top of his head, and would resort to saying someone remotely famous. The joke becomes that much funnier now.
I always wondered why Doc Brown can just walk into the high school in the first movie and have absolutely no one question him in any way. Then I realized that in 1955, he wasn't "insane and probably dangerous loner" Doc Brown, he's Emmett L. "local (and slightly eccentric) millionaire who helped out with the Manhattan Project" Brown. He was initially thought of favorably by the people of Hill Valley, until he put everything he had into his work and was shunned as a result.
He also could've just explained it away that he was showing a relative/new prospective student (or both) around the place, being that handsome and charming young Calvin Klein fellow
There's been a lot of speculation as to what happened to the Marty we see go back to 1955 at the end of the film (Marty II). I puzzled and puzzled until I stumbled across the scene in Pt. III in which Doc doesn't remember dressing Marty in those ridiculous clothes. Why is this relevant? This Doc, much like the Marty we follow throughout the films, is from a timeline which no longer exists, and has memories of it. This Doc does not recall his interactions with Marty set into motion by his own presence in 1885. However you explain it, somehow the mechanics of time-travel in this universe absorb versions of time-travelers with altered pasts into the original one without their memories (at least, as far as we can see) to maintain continuity.
This actually solidifies the idea Doc had about everyone from the original 1885 having no memories of the alternate universes. They aren't different people and they didn't just hop from one parallel universe to the next.
There is far more Fridge Horror version of what happened to him. Marty-2 could be stuck in 1955 which would wrap up his non-presence very neatly. After all, what people need to understand about this version of Marty is that he has lived a much more affluent life than the original. His dad is rich and extroverted, his mum is no longer an alcoholic, his siblings are successful, and he has every material possession that he has ever wanted. A far cry from the original Marty who came from a nearly broken home. Well here's the thing: what was the original Marty doing when he received the flyer concerning the exact time and date of the Hill Valley clock tower being struck? He was staring wistfully at the truck in the dealership window - an act in which Marty-2 would never have needed to do as he already owned it. We can still conceivably have the entire movie play out exactly the way that we see it happen, but no flyer means no return trip. What happened to him at this point enters WMG, but just suppose that he then went to live with Doc Brown and became George's mentor or came up with the suggestion to befriend Marty's younger self in the first place. Makes you think.
With all the changes Marty made in the first movie, how could he and Doc Brown be sure that the time the lightning struck the clock tower hadn't changed too? By noticing that the "Save the Clock Tower" Flyer doesn't fade like the picture does! Of course, giving the current a place to travel might have prevented the clock from being damaged, in effect making it a crude lightning rod.
Marty walks in on Biff while he's watching Clint Eastwood in the hot tub. The scene in particular has Clint be shot several times and being knocked down, before revealing he was wearing a bulletproof vest. Sound familiar?
When 1955 Doc views the video of himself from 1985, he remarks, "Thank God I've still got my hair!" It does seem strange that his hairline is at exactly the same level of recession in 1955 and 1985. (In contrast, Christopher Lloyd himself, 30 years after making the film, is almost entirely bald.) Maybe Doc invented something that halts, but doesn't reverse, hair loss.
Unlikely as the Doc claims he never made anything that works... unless he did intend to make something that let hair grow back, but he failed, and the result was a side-effect of halting hair-loss, meaning that just like the mind-reading machine mentioned above, he didn't realize he had actually technically succeeded, just in a very round-about kind of way.
Doc being the Bungling Inventor that he is, it's very likely that the Time Machine (and its miniature) are not the only creations of his that are Made of Incendium, especially if his track record is consistent failure; he need not be successful to have burned off all his hair, and may in fact be surprised that he managed not to lose the rest of it for the next 30 years.
The reason that Dave disappeared before Marty is simply that the ripple effect worked from the moment things changed in the past up to the present, meaning Dave, the eldest child, was the first to fade out, giving Marty more time to right what went wrong.
What exactly WAS Doc's experiment with the clocks all being 25 minutes slow? Well, remember how Einstein's watch was 1 minute slow because he'd been sent one minute into the future? Doc might be crazy, but he wouldn't put his beloved dog in danger by only testing the time-travel science on him alone. The clocks must have been some prior experiment, possibly testing the time-travel abilities of the Flux Capacitor itself, or maybe just testing sending inorganic matter through time and having it arrive unharmed. It's possible that the Delorean was more of the final stage of the project, when everything was guaranteed to work. He does seem pretty certain that the Delorean won't hit Marty and himself at the mall.
Of course, the theory that the Doc somehow sent his whole lab through time does run into Fridge Logic: how did he reach 88MPH in a garage? This is a fundamental law of Back to the Future time travel and seeing as we have established that even a 1 minute jump requires 88 MPH, the idea that a 25 minute jump somehow wouldn't is a pretty problematic leap in credulity.
A bit of a meta example - the first film was clearly designed to provide a nostalgic view of 1950s-era America and much Lampshade Hanging is done pointing out how much things have changed from the 50s to "present day". Except "present day" was in the mid-80s, meaning people reading this are now as far removed from the setting of Back to the Future's "present" as its original audience was from its "past" setting. Hilariously, the film is so jam-packed with 80s pop culture, from fashion to music to technology, the segments of the film set in "the present" are now just as much of a nostalgic throwback for children of the 80s as the past segments were for children of the 50s.
The writers showed a great deal of foresight in realizing this possibility for the 'Cafe 80's' scene in Part II, which may be seen as a throwaway gag but is actually a pretty good prediction of 80's nostalgia while the 80's were still going on.
It seems odd at first that after the Libyans crash that we never hear from them again, but they were traveling between 60 and 70mph in a vehicle not known for its safety with one of the two hanging out of the open sun roof with an RPG-7 rocket launcher. RPG-7 grenades have impact fuses without any additional safety devices. If the impact with the photo-mat didn't kill them both outright, the blast from the RPG would have surely finished them off.
Marty's original family were a bunch of losers. His brother works in a fast food joint, his sister is apparently completely cynical and jaded and his dad is working some low level white collar grunt job while being bullied by Biff, his superior, into doing Biff's own work as well, meanwhile his mother's overweight, an alcoholic and has an apparently spiteful attitude to anybody else being even remotely happy. The house is also a craphole. After he returns to 1985 they're completely different. His brother now has some kind of office job, his sister is now more socially outgoing, his parents are Happily Married and successful, the house is nicer and now Biff is working as a car cleaner (presumably, George is doing the job he helped Biff into himself). Some people have criticized this ending as being too materialistic, but it's really not. Only one small change in the timeline has led to such a dramatic improvement in the McFly family, that change being George learned to stand up for himself and to not give up in the face of adversity. If you pay attention, George has only just got his first novel published, even though he presumably has been trying since the 50s. The original George probably wrote his first manuscript, got a rejection, and gave up because he was trampled down and broken. His kids, unconsciously following his example, are the same way. The new George probably wrote his first manuscript, got a rejection, and started work on another better manuscript. That also got rejected, so did the next one and the next one. But he didn't ever give up and by 1985 his persistence has finally been rewarded. In the meantime he was probably doing the job Biff was originally doing because he was clearly good enough to do it. His kids, unconsciously following his example, are the same way.
After Marty gets hit with his grandfather's car in 1955, he wakes up to Lorraine telling him he's been asleep for almost nine hours. He doesn't look too banged up to have passed out for that long. But then you remember that Marty had to meet Doc at 1 AM the night he went back in time, without much more than what looked like a cat nap after dinner. He'd have probably been awake for over twenty-four hours real time, not to mention mentally exhausted from what was happening, and was sleeping it off (there's the Reality Subtext that Michal J. Fox was only sleeping two hours a night at this point, splitting his time between filming both this movie and Family Ties).
Fridge Heartwarming: Marty tells 1955 Lorraine that she might regret drinking later in life when she takes a swig from her mother's personal liquor stash. 1985 Lorraine, she's a bloated alcoholic, so he knows for a fact that she's going to regret it. This also makes his discovery about her smoking even funnier: he expects her to drink and is shocked that she sleeps around, but smoking warrants aSpit Take.
In 1955, Marty carelessly informs Goldie Wilson that he's going to be mayor. While this was already going to happen by 1985, it's possible that Marty's words encouraged Goldie a little more. While this has already been brought up, think of the town in this case. In the original 1985, there's a porno theatre on the main street, Lyon Estates looks like a slum for low-income families, and the town is plagued by the machinations of the local mad scientist Emmett Brown. Now consider the town in the later movies, with George McFly, local author winning an award, and local mad scientist Emmett Brown commended for his work in the field of science. Not only that, but while the porno theatre is still there in 1985, the town is obviously thriving by 2015, with a sprawling mall, a lake in the middle of the town, trendy shops everywhere, Goldie Wilson III promoting hover-conversions for cars (possibly something he himself invented)... overall not only has the town's condition massively improved but it may all have been the work of Goldie Wilson II becoming mayor and, as he said at the time, 'cleaning up this town'. Maybe Marty's idea of him being mayor encouraged him to try even harder than before?
More Fridge Heartwarming courtesy of the comic: in the "Who Is Marty McFly?" arc, Doc is forced to gun down a series of animatronic copies of Marty to protect the real Marty. Since they had both initially believed that these were versions of Marty from other timelines and Doc has only just figured out the truth, he hastily assures Marty that it's not what it looks like, explaining, "I would never hurt you, no matter what timeline you came from." With this in mind, weand theyprobably should have realized the truth earlier when one of the "Martys" socked Doc in the face, even when he was trying to reason with them and offer a solution to the problem. The real Marty wouldn't hurt Doc either!
From the Telltale game: Emmett's invention in 1931 was basically a flying car. If we take that game and The Ride (where Doc founds the Institute of Future Technology) together, we can surmise that the only reason we didn't have ubiquitous flying cars by the Real Life 2015...is because Real Life didn't have Doc to invent them.
After Marty returns to 1985 and Doc gets shot, he doesn't get up and reveal he's alive until after the other Marty goes back in time and the Libyans crash. While it's possible that the attack knocked him out for a couple of minutes, there are two other perfectly good reasons not to get back up immediately: first, by playing dead, he lets the Libyans think they succeeded in killing him and that they don't have to come back for a kill shot. And second, by making Marty think he's dead, he preserves Marty's memories of the incident and prevents a potential paradox.
Another possible reason: We know that timeline changes take a while to take place. It's possible that when Marty ran up to him, Doc was dead...and then the ripple effect kicked in.
In the "Time Served" storyarc from the comic book series, Linda seems to have a poor opinion of Biff, even though he's appeared to be nice to the whole family since George knocked him out (and the kids at least shouldn't have experienced mean Biff). But Lorraine probably told the kids the story of how she and George got together many times (like she did in the original timeline), and while Lorraine and George are now on good terms with Biff despite the Attempted Rape, Linda likely thinks of it as the serious issue that it is.
Additionally, in that same arc, Lorraine still sees good in Joey despite having gone to jail, saying that when people make mistakes they become better people. It's possible she had this mindset towards Biff after he got knocked out - he was a horrible person but was taught a lesson and became a better person (at least to George and Lorraine).
Shortly after meeting Marty, Lorraine's father calls him an idiot. Now this could be due to him letting slip stuff about the future which doesn't make sense to them. But then, think about it. If Marty had any idea what he was doing, he would have been watching what he was saying from the beginning. His acting without thinking even kick started the whole issue with changing the past in the first place, even though he knew that George getting hit with the car was what brought his parents together. Finally, we learn in the sequels he effectively ruins his life TWICE due to his pride. Lorraine's father had it right. Marty is an idiot.
Doc Brown made the deal with the Libyans to obtain plutonium in 1985. Perhaps the Libyans behind the Pan-Am Flight 103 originally planned to detonate a nuke aboard a plane and Doc cheating them set their timetable back 3 years.
Biff attempted to rape Lorraine in 1955.
And it's Marty's fault!
In the end of the Part I movie, Biff still is around the family... as some kind of friend! They even acknowledge if it weren't for Biff being a jerk and an almost rapist, they wouldn't be together.
I don't think Biff is a "friend" of the McFlys'. He's only there to work on their cars, as part of his auto-detailing job. Biff's just being a massive suck-up to everyone because he's terrified of George now, and is thus trying to be on his best behavior around him.
In the 1985-A of Part II, they're married and there's obviously no love lost between them. When Lorraine-A tries to leave, Biff-A threatens her kids, which means he's been holding that over her head. In other words, he's been raping her since they got married.
Also, Lorraine-A says to Marty "Oh, they must've hit you hard on the head this time", making us realize how particularly horrible Marty-A's childhood must have been in this time.
To take this one step further: don't forget that the Biff of 1985-A murdered George McFly in 1973-A. It's entirely possible that Lorraine witnessed this, and was finally coerced into marrying Biff (and prevented from reporting him to the police) by the threat that either she or one of her children may be next. What a charming guy.
Looking at her reaction during the newsreel, Lorraine isn't exactly a beaming bride.
After Lorraine attempts to leave, listen to her words. She's using classic abuse victim phrases to excuse Biff's behavior, stating that "she deserved it" and "he looks after us, he deserves our respect." Despite her (quite refreshing) outburst when she threatens to leave, Lorraine is a broken woman by this point, and it's positively gut-wrenching to watch, especially if you have an experience with abuse personally. Then, compare her in 1955, where she tells Biff she wouldn't marry him "even if [he] had a million dollars," before kicking him in the shin and slamming him over the head with her dress box. How this feisty girl became the broken woman of 1985A is best left unexplored.
We've seen how terrible Biff treats Marty's mother in 1985A, and it's heavily implied that she's being raped by Biff. It's horrible in on itself, and then you remember that Marty has a sister. Given that Biff has complete power over the McFlys, imagine what Biff could be doing to Marty's sister.
You know how people ask if "You kiss your mother with that mouth?" when you swear? Well, early in the first movie when the McFlys are eating dinner, Marty's brother Dave lets out a loud "Damn!" when he realized he was late for work. His mother then proceeded tell him not to swear and then asked him for a kiss on the cheek.
Marty's mom asks his brother to "kiss your mother before you go". She means a kiss on the cheek, of course. Later, in 1955, Marty gets an actual kiss from his mother.
That's also a case of Fridge Brilliance: People have the ability to unconsciously detect the genetic viability of potential offspring in another person. Sometimes it's by way of pheromones and other times by saliva. This is one reason why two people in Real Life just might not be the right "type" for each other. So, when Lorraine kisses Marty, she gets biologically "turned-off" because she senses his genetic material mixing with hers wouldn't make for a very healthy kid.
Maybe some people have that ability, but surely not all. There have been cases where siblings were separated at a young age, then later met as strangers, fell in love, and got married. And they didn't realize anything was wrong until years later when they decided to trace their family histories...
Incest and inbreeding isn't quite as dangerous as the 'redneck stereotype' suggests. If the family has been incorporating new genetic material on a fairly regular basis, even a brother and sister may not be incompatible enough yet to set off the woman's genetic viability detection system. Problems don't start to show up until incest has been pervasive for a few generations.
This is more of a call back to Lorraine's apparently oft-retold story about how she knew George was the man for her when she kissed him. Since that was true in both timelines, kissing Marty may have given her a strong signal it would end badly. They padded it with a joke the audience was in on with the brother bit.
In Back to the Future, when Marty returns to 1985, a lot of things are different than when he felt, including his girlfriend's appearance. The meta explanation for this is a simple case of The Other Darrin. However, it becomes horrifying when you consider a possible in-universe explanation. What if his own parents weren't the only teenage couple whose meeting Marty disrupted? Maybe, during the chase with Biff or the Johnny B. Goode recital, Jennifer's parents were in the crowd and were too busy watching Marty to meet each other? Than, at a later time, one parent or the other got married to someone else and had a daughter named Jennifer with them. So the reason Jennifer looks different when Marty gets back is that she has one different parent than before, and is essentially a completely different person.
But they both happen to look just like Claudia Wells?
Since the second Jennifer had one parent in common with the first one, it makes sense that she would look similar.
There are fully armed Libyan terrorists freely driving around America. Fully armed Libyan terrorists planning to build a dirty bomb and who were in no way defeated by the end of the movie.
Not defeated? They crashed into a Photo-fox kiosk at upwards of 70mph. Seeing as how neither were wearing seat belts and were driving a car where the only crumple zone is one's face, the only place they are going is the hospital or the morgue.
Speaking of which, a man in possession of stolen nuclear material has plenty of time to casually pack his bags for a trip after a gun battle at the local mall?
When Marty accidentally prevented his parents from falling in love, they probably would have both gone off to marry other people, meaning that Marty erased at least two other children from time itself in order to ensure his own existence.
People can get married and not have children, y'know, the wedding isn't what makes the babies. Not to mention that Lorraine at least would probably have ended up marrying Biff if Marty hadn't gotten his folks back together.
Even if it's true that George and Lorraine married other people and each had at least one child, there were at least three children who were not born because of this. And if the people they married were supposed to marry and have children with someone else then that's more children that don't exist. And the original spouse of the people who now married George and Lorraine might have married people who were supposed to marry someone else and have children themselves and so on and so on. Who knows how many people might suddenly have not existed because George and Lorraine didn't get together? And there's no reason to think that it better to let the alternate universe children exist at the cost of all the original universe children.
Plus, we can probably forgive Marty for being more concerned over the fate of his own actual siblings rather than hypothetical alternative history children who may-or-may-not exist in some alternative future he's not even aware of. The guy has trouble thinking fourth dimensionally at the best of times, and he doesn't learn that changing the past creates alternative futures for proper until the next movie.
We all know how much of a Crapsack World Biff made when he screwed with history. Imagine if the Libyans discovered the time machine...
Actually probably nothing. There really is no conceivable reason why they would get in that car and speed off at 88 MPH instead of just taking back the plutonium and escaping in their van. And even if they did, it's very likely they would have been shot pretty quickly in 1955 - maybe even by Old Man Peabody given how he was packing a shotgun and had the element of surprise.
In order to have got into the USA without attracting too much attention, at least some of them would have had to know English. If one of them had, say, looked at the little notebook Doc was scribbling on, or noticed the big "Destination Time" and "Last Time Departed From" inside the car, well, there were four Libyans, and they had an assault rifle and a rocket launcher. They could have caused serious havoc in 1955, and that's assuming they didn't deliberately go further back in time...
Or they could have temporarily caused serious havoc in Hill Valley in 1955 until they were taken down by the citizenry. Assault rifles and grenade launchers have existed in very close to their present form since the World War II era, and the kind of serious gun control laws that brought the term "zip gun" into prominence in the 1980's weren't enacted until the '60's, so it's extremely unlikely that Old Man Peabody was the only armed citizen in town.
They more than likely wouldn't just go back to 1955 and cause some chaos. They'd go back in time where major events took place, and remake the world into their ideal. So it'd be even worse.
Again though, this whole scenario revolves around them taking the time to read Brown's notes instead of just leaving with their stolen plutonium after starting a gunfight in a public place. They are not going to care enough about the DeLorean to investigate anything because there is no legitimate reason for them to be interested in it. And when the police arrive and find the corpse of notoriously crazy Doc Brown who was shot whilst testing out a time machine? They are going to call the bomb squad to blow it up once they realise that there is a nuclear reactor on the back of it. There is no danger of the time machine falling into anyone's hands.
In the first film, the 1955 Doc is amazed to find out an invention of his, namely the time machine, actually materialized and works in the future. Then in Part III, after he successfully uses it himself to send Marty back to 1985, he discovers a letter by his 1985 self explicitly telling Marty to destroy it immediately after returning to 1985 himself. It had to have shocked him that he'd go from pride to regret over the time machine.
Cracked points out some new disturbing implications, like, Doc burning down his house for the insurance money, the fact that there were bystanders when Biff was molesting Lorraine who didn't try to intervene, new implications about the deleted scene between Biff and the former mechanic Terry, the fact that the "sleep-inducing alpha rhythm generator" clearly is an electronic version of a roofie, the revelation that millions more people died in 1985-A because the Vietnam War went on longer (as noted by a newspaper article), and just how many people had their futures altered because of the locomotive theft in part III.
In addition to no one wanting to mess with Biff, this is 1955. Everyone surrounding them probably thought that she deserved it for her low-cut dress and her loose attitude to sex and drink. Values Dissonance at its finest. We also have no idea where Brown got that Alpha-rhythm generator from. Given how he is a man quite happy to steal plutonium from terrorists it is not unreasonable to suggest that he stole it from somewhere that would have legitimate use for it such as a hospital, a dentist, or even an old-people's home as an alternative for sleeping pills. Remember that almost everything has at least one malicious use in the wrong hands no matter how useful it is.
Lorraine was a victim of attempted rape by Biff Tannen, whom drops by at the McFly household every now and then. Imagine how uncomfortable it must feel for Lorraine to be anywhere near the person who almost raped her.
Or how incredibly satisfying it must be to watch her former bully and rapist cower every time her husband walks into the room. Everyone keeps assuming that it was George and not her that hired Biff, but not everyone responds to trauma in the same way.
The comics also reveal that the Doc of 1985-A, whom we were told was sent to a mental institution, was also lobotomized. That's bad enough in itself, but the Marty of that timeline was shipped off to boarding school by his new stepdad. Does he know? Does he not know? Will he ever find out? There's no non-horrifying option.
There is a good chance that Marty-A has no idea who Doctor Brown is, especially if one ascribes to the theory that the original deliberately hunted down Marty thanks to knowing about the pre-destination paradox.
Judging by Doc's excitement when he successfully sends Einstein 1 minute into the future, one gets the impression he wasn't completely sure it world work. However he positioned himself and Marty right in front of a car speeding towards them at 88 miles per hour!
Why does getting erased from time seem to be somewhat agonizing? It's likely because they're literally losing the nerves and small body partsnote That the camera doesn't focus that they need to function. We see this happen when Marty starts to lose his hand.