In TBBT, Sheldon tells Penny that the reason he knocks three times is because when he was thirteen he opened his parents’ bedroom door without knocking once and saw his father in bed with another woman. So, he instituted a third knock.

We’ve all been waiting for this moment, especially since George’s emotional affair with Brenda Sparks. In the preview for this weeks episode, we saw Sheldon opening a door and looking shocked by what he saw. I have a feeling that is the moment in question.

If it were a few seasons ago, this moment wouldn’t surprise me if it was outright what older Sheldon said it was. But George and Mary’s relationship doesn’t seem to be in the tenuous place it was a few seasons ago and, especially with them writing letter back and forth while Mary is in Germany (which I thought was just precious.)

Not to mention the fact that the George talked about in TBBT is NOT the George we’ve gotten to know in Young Sheldon.

So, here is how I think the writers will avoid an outright retcon while still maintaining George and Mary’s marriage. I think that Mary will do something to change her appearance. Maybe she will cut or dye her hair, impulsively. Say it is a bit of a midlife crisis sort of moment. So, Sheldon does walk in on his father in bed…but with his mother. He just doesn’t realize that it is Mary and leaves before he can. Out of embarrassment on both their parts, George and Sheldon don’t mention it and so Sheldon thinks George has been unfaithful.

I do hate that, no matter how it is handled, this will negatively impact George and Sheldon relationship (one of my favorite dynamics in the show) but it won’t negatively impact Mary and George’s relationship and it’ll give the audience some dramatic irony.

Disclaimer: I am well aware that I could be totally wrong.