I think it was the episode where Lisa kills Snowball II, and then gets and kills a dozen more cats before finding one that looks like the original Snowball II and pretending nothing happened. After previous episodes about the pets that were relatively heartfelt and treated them well it seemed unusually cruel and South Park esque. This alongside repeating the gag from the Principal and the Pauper, already a bad and unfunny episode, and then actually calling out the parallels by having Skinner show up and remark on it, indicated to me that the shift in tone and style was probably not a temporary thing. I don't know how many years it's been since then, but it seems my conclusion has been borne out.
Still, we'll always have the early seasons, and for that I'm grateful.