I’ll go with the low-hanging fruit: Mein Kampf. I’ve read it, cover to cover. As a piece of propaganda, it’s good. As an example of good writing? Absolutely not (though I will admit I have only read it in translation). Oh, and the whole fascist, racist, and generally shitty worldview of the author that he infuses into the text. And the fact that the author is literally Hitler. You 5-star that book? You’re a Nazi. Period. And as a Jewish person, I don’t look too kindly on them.
Catcher in the Rye, because they probably related to Holden Caulfied.
Catcher in the Rye but usually it means they haven’t read a book since high school and aren’t being upfront about it.
Holden is literally me, 5 stars.
What if they note they only own the one copy?
This one is a yellow flag but it heavily depends on why they liked it, I would have to check