Certain minor peculiarities of language are used unconsciously by poets.
Goal
Conclusively figure out which author wrote a given poem.
Plan
Find in that poem an idiosyncrasy that has previously only appeared in the work of poet X, to then surmise that poet X wrote this one too.
Evaluate
The question stem assigns us a goal of showing how this test, or this plan, would fail to achieve its desired result.
So, we should consider any potential exceptions or weaknesses in this "fingerprint" approach.
Goal
We need a way to argue that we could do this plan but fail to achieve its goal.
We could find a poem with unknown authorship, spot an idiosyncrasy (a weird choice, like when Kanye first started auto-tuning vocals, or when Dr. Seuss makes up new species of animals to create a lazy rhyme), match that idiosyncrasy to a known poet, but still be wrong in thinking that this poet is the author of the unknown poem.