I noticed that Ralph was Flanderized pretty hard by season 6. He was kind of dumb up until this point, but everything Ralph has to say for a full episode revolves around his cat, regardless of context. He has now metamorphized into a full on smooth brain. This obviously isn't zombie territory yet, but there are notable omens/symptoms of it early on.
This is an interesting case study of where exactly the show starts its decline. The Principal and the Pauper is the one everyone points to as the turning point, but I'm of the opinion that decline begins in bits and pieces rather than as a whole, since if something performs good overall you're not likely to notice the small things beginning to slip.