The Big Bang Theory and Young Sheldon: Sheldon is mostly just a liar and a jerk. His obliviousness to his own faults and lack of empathy mean he tends to rewrite his own recollection of events with his opinion of those involved, while Young Sheldon shows how things actually happened.

For instance:

• Despite Sheldon's stories, his father was a kind and supportive man who made one mistake (or so Sheldon thought. He walked in on his parents roleplaying and Sheldon didn't recognize his mom because she was wearing a wig and speaking in a German accent, leading him to think that his dad was having an affair), and that mistake warped Sheldon's opinion of his father to only focusing on perhaps the few minor mistakes he made in his life. It turns that his mother Mary was the bad spouse and parent and she turned Sheldon into the jackass monster he grew up to become.

• He describes Billy Sparks as a bully, even though (outside of the pilot episode) Billy was always nice to Sheldon while Billy's younger sister Bobbi was the one who actually bullied him.

• Also because of Tam unintentionally hurting Sheldon, he leaves him out of every story he tells about his childhood in Texas.

• This also extends to his family. Due to his great relationship with his grandmother he ignores the many flaws she had during the prequel, while with Georgie and Missy he can be cruel in how he talks about them and all the times they "bullied" him growing up, even though he was the one instigating 90% of the time (one episode showed Sheldon befriending one of Georgie's bullies and using him to get what he wanted, and most of Missy's worst behaviour towards him was caused by how horrible he was to her).

Sheldon was the bully to everybody just like he is as an adult!

And then in the Young Sheldon finale:

• The adult Sheldon has been an unreliable narrator throughout the series, but here Amy calls him out on it when he mentions how even at a young age he was constantly doing things to make other people happy, which she claims is a lie since he never does that and Amy says “You’ve got to be kidding me”. This means Sheldon is a deluded narcissist.

• After years of making his friends put up with things they didn't want to for his sake, it's shown that Sheldon's kids are doing the same to him. Sheldon won’t even go to his own son Leonard’s hockey game because he doesn’t wanna see children hitting a rubber puck with a stick and Amy has to make him go and also to make him wear a supporting jumper for their son’s team. Sheldon is actually annoyed that his son is a hockey player and also says Leonard knows Sheldon loves him, despite his "foolish and embarrassing hobby" of hockey playing, to which Amy points out is not a hobby.