Other mistake: When Hank tells John Redcorn that his family is not Jewish, John is noticeably surprised. However, in the episode "Revenge of the Lutefisk" (3x21), John was at church with the Hills and Gribbles. He already knew that they are not Jewish.
Bad Girls, Bad Girls Whatcha Gonna Do - S7-E3
Other mistake: When Bobby is caught spray-painting the fence, Hank says that he and Peggy took their wedding photos in front of that fence. In the episode "As Old As The Hills", part one, there is a photo of Hank and Peggy living in an apartment after they got married.
Other mistake: Dale laughs at Nancy for picking up the diazinon, instead of the bird repeller gel. He then picks up a bag, goes up the ladder, and when he turns the bag around, it says diazanon on the bag he picked up. The only gel in the scene was what Nancy picked up.
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.