Side story about another use of that cartoon. A friend in grad school was taking his Graph Theory qualifying exam. One question was the old “Here is a new property graphs might have. Here is a proposed theorem. Prove it or provide a counter-example.”

As we were having a few drinks post-exam, he mentioned it and said that he had written (some large number of pages, the exact value escapes me) on it, laying out lemma after lemma in an attempt to prove it in steps. He said when he got to the meat of the work there was one detail he coudn’t get over, so he finally wrote “The proof of this trivial observation is left to the interested reader.” He then used the tools he’d developed to prove the statement.

You guessed by now, I’m sure, the idea was false and there was a trivial counter-example (trivial once you saw it, apparently). The faculty members who read his exam taped a copy of that cartoon to his “interested reader” comment. (He was given a passing mark on the exam.)