Speaking for myself, I definitely had some instances of the former. But I was relieved to learn years later than "Cow Tools" was an instance of the latter.
I'd say, with the utmost respect for Larson, that this isn't so much one you're too dumb to get as it is a rare flub in his impressive oeuvre!
For me this comic combined with the general limitations of what he does are what make him second to Bill Watterson. Somehow Watterson managed to be much broader and yet as far as I know there's not a single flub in any of his Calvin & Hobbes comics.
Anyway, here's the pic alone https://upload.wikimedia.org/wikipedia/en/9/9a/Cow_Tools_car...
And here's the wiki article it's in, with explanation, such as it is https://en.wikipedia.org/wiki/Cow_Tools
Unrelated, look up the wiki article for Thagomizer. It's funny and show life following art (Q: are Larson's comics art? A: yes.)
Edit: save you the effort https://en.wikipedia.org/wiki/Thagomizer