Sheldon was often paranoid about Penny pulling one over on him in The Big Bang Theory despite also thinking he was smarter than her, an internal contradiction that Young Sheldon offered an answer for via Ming-Na Wen's cameo. Sheldon Cooper and Penny were always bound to butt heads on The Big Bang Theory. Penny was free-spirited and fun-loving where Sheldon was uptight and cautious, and Kaley Cuoco's Big Bang Theory heroine was also deeply unpretentious where Sheldon was often insufferably smug about his superior intellect.
However, one of Sheldon's character flaws always jarred with his stated view of Penny. Throughout The Big Bang Theory (particularly in earlier seasons before he softened up somewhat), Sheldon is often suspicious of Penny and distrusts her motives. This should theoretically jar with the fact that he thinks himself far smarter than her, making his conviction that she can fool him seemingly misplaced.
However, Ming-Na Wen's Carol Lee outsmarting Sheldon in Young Sheldon season 5 explained this The Big Bang Theory character detail. Much like one Young Sheldon season 5 episode justified Sheldon's obsession with his spot by illustrating his struggles sharing a room with Missy, Carol Lee's role on the show explained why The Big Bang Theory's version of Sheldon is so oddly suspicious of Penny while still patronizing her intellect. By booby-trapping her office and catching Sheldon in the act of trying to sabotage her work, Lee proves that the character had a format experience of a glamorous, charismatic woman wooing his co-workers and tricking him into exposing his intellectual insecurities — thus making his paranoid distrust of Penny make sense.
Throughout the action of Young Sheldon's "A Free Scratcher and Feminine Wiles" (season 5, episode 14), Lee plays Sheldon's two competing colleagues off one another and ensures the pair of older scientists don't come to blows. However, this earns the ire of Young Sheldon's hero himself, prompting Sheldon to hatch a plan and get revenge on her by sabotaging Lee's work. Being a veteran in the field, Lee has encountered plenty of petty colleagues before and preempts Sheldon's attempts by booby-trapping her office and thus catching him red-handed.
The fact that Sheldon fails to acknowledge Lee's impressive intelligence in the episode and instead accuses her of using her feminine wiles to beguile his co-workers proves that the character's hubris can lead him to see even women who he doesn't respect as a threat to his work. This would explain why Sheldon is so suspicious of Penny in early The Big Bang Theory outings before eventually warming to her, as the character held onto his conviction that Lee outsmarted him rather than her simply being both his professional and personal equal in matters of both guile and scientific prowess. While many sitcom spinoffs retcon earlier character details, this Young Sheldon outing succeeded in setting up a Sheldon character flaw that can be seen throughout The Big Bang Theory's early seasons.
