Young Sheldon season 6 stealthily included an Easter egg that tied Mary's secret boyfriend to Penny. CBS made a tremendous effort to tie the prequel to its flagship series during its early years, especially as The Big Bang Theory neared its end. As years went on, however, Young Sheldon started to establish its own identity by tackling stories that have no bearing on what happened in The Big Bang Theory. This doesn't mean, however, that CBS stopped incorporating details in Sheldon's origins show that are secretly in reference to its parent sitcom.
As Young Sheldon expanded its storytelling, there has been an increased focus on the rest of the cast. Instead of weekly episodes only following Sheldon, all the members of the Cooper family are given their respective narratives. This made it easier to also root for various members of the clan, not just the boy genius. Oftentimes, Penny is compared to Missy, as Sheldon's twin sister shares many personal traits with Leonard's wife. However, in an unexpected turn of events, however, Young Sheldon season 6 made a parallel between Penny and Mary, albeit in the most subtle way possible.
In Young Sheldon season 6, episode 3, "Passion's Harvest and a Sheldocracy," Mary was getting used to not being tied up with work in her church after she was forced out of the congregation due to Georgie and Mandy's unexpected pregnancy. As she tried filling her newfound free time, Mary got engrossed in erotic romance novels, which resulted in the Cooper matriarch having naughty dreams with her made-up boyfriend, Dusty, played by Michael Trucco. Long-time viewers of The Big Bang Theory may find Mary's fictional lover familiar because he previously appeared in the nerd-centric sitcom, albeit playing a different character.
Trucco played Dr. David Underhill in The Big Bang Theory season 2, episode 11, "The Bath Item Gift Hypothesis." Underhill was a MacArthur Genius Grant recipient, who was dating Penny in the outing. Considering that the offering also featured the iconic moment where Sheldon received the signed Leonard Nimoy napkin from Penny, resulting in their first hug, it was easy to forget Underhill's existence. In any case, he never appeared again in the show beyond that, as Penny broke up with him after the revelation that he was secretly married. Still, it's a fascinating casting choice to bring him back as Mary's perfect boyfriend in Young Sheldon.
Trucco isn't the first person to play two different in the Young Sheldon and The Big Bang Theory. Lance Barber infamously played Leonard's bully, Jimmy Speckerman before portraying George Cooper in the prequel. This is widely regarded as a plot inconsistency, with neither of the show providing any explanation thus far.
However, just like how Teller playing Amy's dad in The Big Bang Theory and Pus in Young Sheldon isn't a plot hole, Trucco portrayed a fictional character in the spin-off. This means that Dusty doesn't really exist in the universe. Instead, he was a figment of Mary's imagination. In any case, Sheldon's mother also never saw Underhill in The Big Bang Theory, which is why CBS can get away with not acknowledging how much they look like each other.