(Strip Number 5128, Original Publication Date, 12 May 1989)
Panel 1: A little while later, Dr Schell tells Elly and Liz that he did some exploratory work and shows them what was cutting into Farley's leg. Elly is baffled and asks him where the elastic band he's holding came from.
Panel 2: The two of them are aghast when he explains that someone put the elastic band on his leg and forgot and that the result of said brain failure is that the blasted thing worked its way clear to the poor animal's leg bone.
Panel 3: Elly cannot believe it and asks who in the world would do such an awful thing.
Panel 4: When a panicked Liz muffles a cry of alarm because she just remembered where that elastic she lost was, we kind of have an idea of what kind of nitwit did that awful thing. Judging from the way the veterinarian and Elly are staring at her, so do they.
Summary: This is, of course, not the result of active malice on Liz's part. It's the result of passive imbecility that is not learned from because John makes a stupid aside that's meant to end the conversation. Also, one has to have a functional brain to learn from the past and that's not something Liz has.