Garfield is put on a diet where he is given half of a lettuce leaf for a meal. While this was likely done for comedic effect, cats are carnivores and require animal protein to maintain their health. Putting a cat on a veggie-only diet would make them seriously ill and is not something a veterinarian would ever recommend.
All the butter on Jon's face and hands disappears when he puts the turkey in the oven.
When Liz says "What's that?" when she hears the chainsaw from the kitchen, she says it in Grandma's voice.
Liz notices that Garfield hasn't lost any weight and deems his diet to be ineffective. However, he had only been on it for a day, and even if he took it seriously, he wouldn't have lost a significant amount of weight within that time, and if he did, that would've been a greater cause for concern.