If you knew a book had purposeful errors, would you still read it?
Well, guess what -- you already have. Hahaha!
That was kind of a mean trick question. You see, every single book has purposeful errors.
Let me explain. There comes a time in an author's life when he or she will face copyedits (copy edits?). For me, that time was this week. Copyeditors (or copy editors? I DON'T KNOW ANYMORE) are brilliant, detail-oriented people who not only spot and fix typos, grammar mistakes, and spelling errors, but also serve as continuity experts (comparable to script supervisors in TV and film, which means they notice if your character is wearing a green sweater at the beginning of the scene but a purple one by the end).
In short, they save authors from super embarrassing mistakes and also help prose shine before books get published. They're invaluable.
Sometimes, however, writers choose to keep a sentence or word that's incorrect because it suits their character's voice, either in actual dialogue or in narration.
For example, the main character of BRUISED, my YA novel, is a 16-year-old girl who loves martial arts more than anything. She struggles to make Cs in school, so it wouldn't make sense for her to speak with proper grammar all the time. This is not to say all teenagers spend their days spouting purposeful errors; a different teenager in a different book would speak with correct grammar but my girl Imogen's not one to say "whom."
So next time you read a book and think, "That's flat-out wrong! How did that slip by?" It didn't. It was noticed, possibly agonized over, and deliberately kept.