One of the biggest Young Sheldon inconsistencies is its depiction of George Sr., who was described quite differently on TBBT. Sure, Sheldon's dad George clearly loved his beer; he's usually seen having one, maybe two every night, and sometimes three on Fridays. To a young Sheldon's eyes, it might have seemed that his father drank a lot. So, perhaps it isn't an inconsistency insomuch as it is a young boy's perception being carried over to his adult life.
This was even proven, to some degree, when Sheldon unearthed an old VHS tape of his father giving a pep talk during an episode of The Big Bang Theory and realized with the help of Amy that sometimes things can be "observer-relative." Who knows, maybe the drinking got worse over time, and viewers have yet to see it. Either way, it seems like Sheldon's dad was a man who drank a lot of beer but didn't really have a problem with alcohol, per se.
Another one of the bigger Young Sheldon inconsistencies that the spinoff changed was Sheldon's love of scheduling and rules. A season 5 episode of Young Sheldon reveals that the titular character heavily regulated other students' use of his dorm room at East Texas Tech, though he hated scheduling these things. Given Sheldon's meticulous nature and love of routine, one would assume that he would have no problem with this.
Additionally, Sheldon's extreme adherence to routine was a major part of The Big Bang Theory, to the point that he once revealed his bowel movements ran like a "German train schedule." While it's possible that he developed these idiosyncrasies later in life, it's tough to believe that Sheldon would, at any point, have reviled creating a schedule for himself and his cohorts.
Viewers have seen glimpses of how Sheldon was picked on at school, but it seems as though he mainly kept to himself and wasn't really tormented and bullied as much as he describes later to his friends on The Big Bang Theory, only adding to the list of Young Sheldon inconsistencies. What's more, there are a number of people Sheldon could consider friends on Young Sheldon.
Maybe there are things audiences just aren't shown in the series, and there's still more story to tell. But, from what has been depicted thus far, while Sheldon didn't really have many friends, most of the high schoolers just gave him weird looks and rolled eyes. His biggest bully was the dimwitted neighbor Billy Sparks and the little girl whose bullying seemed to stop as soon as it began.
Tam surely must be hurt to know that Sheldon told all of his friends in adulthood that he didn't have any friends when he was younger, as Sheldon's claim on TBBT that he was friendless is one of the biggest Young Sheldon inconsistencies. Meanwhile, it seems like he and Tam hung out every day, eating together, studying in the library, and sometimes even getting together after school.
A grown-up Tam did appear on The Big Bang Theory, showing that Sheldon sort of blocked out their friendship after getting upset that Tam moved away, so this could explain that inconsistency. That said, Sheldon even, much to Paige's surprise, had a group of friends in college with whom he hung out with to play games like Dungeons & Dragons.
While Sheldon often discussed how his parents' marriage imploded, with his father's cheating and staying out all night, but very little of that has thus far come to light, and his parents' marriage becomes one of the bigger Young Sheldon inconsistencies. Sure, they don't exactly have the perfect marriage, but, for the most part, his father George seems like a doting, hard-working father.
Young Sheldon season 5 did show signs of fractures starting to come to the surface in the marriage and is likely setting the scene for more of what's to come in terms of how Sheldon remembers his youth. Sheldon seemed to have left out a lot about the happier moments from childhood, instead embellishing the bad in his recollections. Nonetheless, Young Sheldon remains one of the best comedy shows told through a child's perspective.
In The Big Bang Theory, Sheldon mentions one time that his mother had to return home to Texas because their house was slipping off the blocks again, which would imply that they lived in a trailer park. However, more Young Sheldon inconsistencies come to light, as they clearly live in a lovely home. So, when exactly did they move to this type of home on blocks?
It could have been after Sheldon's father died, which is when he was 14 or 15. Or maybe it was after George lost his job. Perhaps this means the Young Sheldon story will take a bad turn at some point this upcoming season, and the family will lose their home — or maybe Mary and Sheldon didn't mean the comment literally. Also, their actual house could possibly be "on blocks," a cheaper method of building a foundation versus poured concrete.
Meemaw's character is quite different on TBBT, and her depiction on the spinoff is one of the biggest Young Sheldon inconsistencies. Sure, during her appearance on The Big Bang Theory, Meemaw, played by June Squibb, was a hit, hilariously ragging on Amy. But, there was no resemblance between her and the Meemaw in Young Sheldon, played by Annie Potts, both in appearance and attitude. Though she did present as the true Meemaw when adult Sheldon poured her a whiskey instead of wine and declared "I made it just how you like it — a lot in a glass."
It's possible that Meemaw became old-fashioned and stern as she aged, but the woman seen in The Big Bang Theory seems almost nothing like the version seen in Young Sheldon. It's hard to believe that Meemaw shifted so rapidly from the wild and unconventional grandmother she was 20+ years prior to the wholesome woman she appeared to be in the present day.
Speaking of Meemaw, Sheldon's grandfather could also be considered one of the most glaring Young Sheldon inconsistencies. In The Big Bang Theory, Sheldon credits his grandfather for getting him into science before he passed away. On Young Sheldon, it has presumably been about four years since the grandfather died, which would mean Sheldon was five years old when he lost Pop-Pop. Sure, five is probably old enough to start developing an interest in particular hobbies, so it's quite possible that someone as intellectually advanced as Sheldon got the science bug from that age.
But he never really talks about his grandfather in Young Sheldon, nor about him being an inspiration. Viewers never see photos of him either, both in Connie's house or in Sheldon's. While he is mentioned from time to time, including one episode when Dr. Sturgis (whose absence in chatter on The Big Bang Theory about Sheldon's childhood is a big plot hole from Young Sheldon) tries on his old jacket at a garage sale, upsetting Connie. But Sheldon rarely quotes or references him.
Eagle-eyed fans noticed as soon as Young Sheldon was released that the actor who plays George, Sheldon's father, is the same actor who played Jimmy, Leonard's high school bully, in an episode of The Big Bang Theory, making this one of the biggest Young Sheldon inconsistencies. In TBBT season 5, episode 11, "The Speckerman Recurrence," Jimmy, played by Lance Barber, reaches out to Leonard, and Leonard finally has the chance to stand up to him. Meanwhile, Barber returned to the TBBT family to play George, and viewers noticed. Sure, it was only a small role way back then, but, for the sake of believability, it's something audiences can't unsee.
Sheldon's eidetic memory turns out to be a curse when it comes to Young Sheldon inconsistencies. In an episode of Young Sheldon, the librarian hands Sheldon the self-help book How to Win Friends and Influence People, which he then goes on to read and attempts to follow in an effort to win friends and influence people. Unfortunately, his tactics don't quite work. However, in The Big Bang Theory, which takes place decades later, an adult Sheldon comes across a mention of Dale Carnegie and this book, and he seems unfamiliar with it. For someone with an eidetic memory, this isn't something he would forget.
When it comes to Missy Cooper, there are a lot of Young Sheldon inconsistencies. She shows up a handful of times on The Big Bang Theory and during her appearances she shares several stories from her and Sheldon's childhood. In one of these tales, Missy claims that Sheldon once built a robot to keep her out of his room. Young Sheldon revealed that Missy and Sheldon actually shared a bedroom for a majority of their childhood, with the siblings only getting their own separate bedrooms shortly after Missy entered puberty.
At this point in Young Sheldon, the prodigy has gotten a dorm since moving off to college, and the robot story has yet to occur. It's possible that this is one of those Young Sheldon vs Big Bang Theory moments that won't ever be seen on the series, as it doesn't technically add anything major to the narrative. Regardless, it was a fun memory on the parent series, and the spinoff would've done well to honor it.
Once again, Sheldon's eidetic memory creates more Young Sheldon inconsistencies. The foundational late theoretical physicist Stephen Hawking was one of Sheldon Cooper's biggest heroes. In The Big Bang Theory season 5, episode 21, "The Hawking Excitation," Hawking is invited to speak in a series of guest lectures at CalTech, and Howard is put in charge of maintaining the equipment on his wheelchair. Sheldon uses this opportunity to do anything and everything he can to meet his hero for the first time, but there's one problem: he's already met him.
In Young Sheldon season 3, episode 16, "Pasadena," Sheldon gets an all-expense paid trip to see Hawking lecture at CalTech. While the pair don't get to actually interact with one another face to face, Sheldon never mentions this excursion in TBBT, and acts as though he's seeing Hawking in the flesh for the very first time. Perhaps it was the interaction itself that Sheldon was referring to, but with his eidetic memory, Sheldon should've remembered that he had already seen Hawking once before.