Something I didn't catch before at the end of S03E01 - Out of Town
After Sally finds the TWA pin of the flight attendant Don sleeps with on the business trip, Sally asks to hear the story from the day she was born. Don begins to explain that it was the middle of the night and he had just come home from work. He starts to trail off and disassociate a bit, so Betty jumps in to finish the story and says that Don didn't feel like getting back into the car. I believe this infers that Don was out having an affair the night Sally was born, which is why he came home so late that night and why he is rendered speechless with guilt in the middle of the story.
Don worked so hard to win Betty back after she discovered his infidelity with Bobbie Barrett in season 2, but just one episode into season 3 and the audience sees confirmation that Don is the same piece of garbage he's always been. He becomes completely closed lying there on the bed, weighing the guilt of the recent flight attendant fling against whoever held his attention the night Sally was born. Spanning years of affairs and countless betrayals of the woman who bore his children and ultimately took him back. Such a fantastic subtle clue and window into his psyche.