Do you fail a critical essay for saying it was writing the wrong poet?

If a student had been specifically asked to write about one poet and instead wrote about a different poet, that is a serious error, however, if the essay is a good one which shows insight into the poet that this student chose to write about, I personally would accept it anyway. The real purpose of studying literature, poetry, and the art of writing, would be served anyway, even by an essay on the wrong poet, as long as it is a good essay. A bad essay on the wrong topic would be unacceptable.