This is a story that I heard from one of the postdocs from my university, which in turn heard it from one of the professor at the university (I didn't bother to verify with him as the source seems relatively reliable).

The said professor was a postdoc in some university in the USA a few decades ago, and he was teaching a basic course on group theory. One of the homework assignments had a question of the form:
"Let $G_1$ be the group $\ldots$, and $G_2$ be the group $\ldots$ Prove that $G_1$ and $G_2$ are isomorphic."

One of the papers submitted had an answer "We will show that $G_1$ is isomorphic..." and some nonsense, followed by "Now we'll show that $G_2$ is isomorphic..." and more nonsense.