When the family is at Cousin Eddie's house, Clarke comments on the Ketchup "Real tomato Ketchup" (meaning homemade), but in the scene before this Ellen is inside the house helping in the kitchen, on the table is a bottle of ketchup.
I think this was Clark's sarcasm at work. They were eating Hamburger Helper patties with no hamburger, so Clark was kinda sayin' "Wow, REAL ketchup, not some sort of imitation mock-up of ketchup?" I don't think he was implying that the ketchup was homemade, just that it was actually made of tomatoes (which, believe it or not, bottled ketchup IS made of real tomatoes, and vinegar, of course). I don't think this qualifies as a slipup.