Warning: Contains SPOILERS for Young Sheldon season 5, episode 1.
The Young Sheldon season 5 premiere creates a The Big Bang Theory plot hole involving Leonard (Johnny Galecki). The much-anticipated return of the CBS sitcom finally happened with the episode titled "One Bad Night and Chaos of Selfish Desires." But while it mainly focused on dealing with the subsequent events of George (Lance Barber) and Brenda's (Melisa Peterman) night at the bar, the spin-off still managed to pose new inconsistencies between it and its parent series.
As the only two characters from the original pilot who made it into the actual series, Sheldon (Jim Parsons) and Leonard's friendship was The Big Bang Theory's main relationship. The pair went through a lot over the years and despite occasional arguments, they were able to maintain their bond until the series wrapped up after 12 seasons. Leonard actually deserves most of the credit, since he'd been very patient and understanding of his roommate's many oddities. That includes Sheldon getting very paranoid about his friend's nose operation and trying everything to dissuade him from going through with it, even if the procedure was advised by his doctor.
In The Big Bang Theory season 8, episode 9 "The Septum Deviation," Leonard's physician informs him that he needs surgery because he has a deviated septum. Upon learning this, Sheldon quickly starts panicking, appalled by the idea that his best friend would risk going under the knife. He becomes so paranoid in fact, that he actively tries to convince Leonard that the safest course of action is to just ignore his medical condition altogether. Despite Sheldon crunching numbers to determine the odds of his friend dying, Leonard is firm in his decision to go through with the surgery. Expectedly, everything goes smoothly and nothing bad happens to him. Still, Sheldon's reaction to Leonard's medical procedure is so odd, especially since he has apparently been a proponent of modern medicine ever since he was a kid.
This is revealed in the Young Sheldon season 5 premiere where George is rushed to the hospital after suffering from chest pains while at the pub with Brenda. When Mary (Zoe Perry) is informed of her husband's health scare, she attempts to gather all their children only to realize that none of them are in their rooms. Eventually, Meemaw (Annie Potts) and Georgie (Montana Jordan) find the twins and bring them back home. Once George's status is clear, the kids' grandmother delivers the good news that their dad will be sent home in a few days. Missy thanks God for this welcome development, but Sheldon counters her saying, "thank modern medicine." If Sheldon had faith in medicine back then, acknowledging that it's the reason that the Cooper patriarch could come home after suffering a mini heart attack, why was he so afraid of Leonard going through a relatively easy medical procedure in The Big Bang Theory?
It's difficult to change Sheldon's mind and his ways. Once he decides on something, chances are that he will stick to that notion until the end—unless he discovers something that proves his previous stance wrong. Obviously, a lot of time has passed by the time Sheldon attempts to dissuade Leonard from having the surgery in The Big Bang Theory after having full faith in it in Young Sheldon. And though it appears to be yet another character discrepancy between the shows, there is the possibility that it's a matter of character growth. Sheldon hadn't come to care for anyone as much as he did for Leonard as he faced his operation, and while Sheldon's commitment to statistics and empirical evidence is usually unwavering, it is perhaps the suggestion of his love for his friend that destabilizes him. After all, when Sheldon learns that Penny is considering breaking up with Leonard (in season 6's The Decoupling Fluctuation) he pleads with her "please, don't hurt my friend" while maintaining the pretence of hating change. Seemingly, Sheldon's love clashes with his logic, which may be an explanation for the plot-hole.