Starring: Brittany Murphy, Pamela Adlon, Mike Judge, Kathy Najimy, Johnny Hardwick
I Never Promised You an Organic Garden - S7-E18
Continuity mistake: During the scene where Peggy still finds insects over the tomatoes, she goes to show Hank one of the tomatoes. As she is showing the tomato it has around six or seven flies buzzing around it. The next shot shows Peggy from behind still holding the tomato, and you can see there is not one fly around the tomato anymore.
Peggy: It was like a knife in his heart when she stuck that fork in his back.
The Wedding of Bobby Hill - S3-E14
Trivia: During Hanks's conversation with Bobby when he tells him that he will have to marry Luanne, look at the shelf behind Bobby. There is a Bart Simpson doll there. A few years before this was featured the Hill family was in the crowd at one of the pee wee football games in the Simpsons episode "Bart Star".
Question: How can Dale be so oblivious to the fact that Joseph isn't his biological son or that Nancy cheated on him with John Redcorn considering in one episode Dale actually caught them both in bed together.
Answer: That's the shtick. Everyone in town knows that Dale's wife is seeing John behind his back and that Joseph is obviously really John's son, but Dale is completely oblivious (he's not that smart of a man). In one episode, Dale even says he's not homophobic because 'John Redcorn is gay, and he's one of my closest friends.' Of course there is a fan theory that Dale is completely aware of what's going on but chooses to ignore it out of fear of his marriage ending.
Also, if Dale *does* know about the affair, what choice does he have anyway? Explain to everyone he meets that Nancy is his wife, and Joseph is her son from her boyfriend? If he wants to stay with Nancy, then raising Joseph as his own child is slightly less awkward. Especially while Joseph is younger. I know of one man who did this. It was obvious that his wife had an affair, and "his" younger daughter had been fathered by someone of a different race. An "elephant in the room" situation.
Answer: In addition to Brian Katcher's response: it's supposed to be a joke that Dale is always suspicious about conspiracy theories, mysteries, etc, but he is "clueless" about his own wife having an affair. Of course, as Brian wrote, many fans think that Dale is pretending to be clueless because he doesn't want Nancy to leave him.