Deliberate mistake: As Dale is taking photos of Bobby's ventriloquist doll you can see he is taking the photo from the upper right of where Bobby is standing in his house. However the angle of which he takes the photo from would be impossible as it's to the left of their screen doors and there is no window or see-through material from that angle.
King of the Hill (1997)
1 deliberate mistake in Now Who's the Dummy? - chronological order
Starring: Brittany Murphy, Pamela Adlon, Mike Judge, Kathy Najimy, Johnny Hardwick
It's Not Easy Being Green - S5-E16
Factual error: Boomhauer's car, Ms. Sally, has a Texas flag license plate on it. These plates were not issued until 1992, but Ms. Sally was driven into the quarry in the late 70's or early 80's.
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".
To Sirloin with Love - S13-E20
Question: What is Boomhower's occupation?
Answer: Jeffrey Boomhauer is a Texas Ranger as can be seen in the final episode at the end when he sets his wallet down, it flips open revealing a Texas Ranger badge.