I've been wondering for a long time about what to do if you find a mistake in an advance reader copy of a book. It's made from uncorrected proofs, so it's not surprising to find a mistake. The real issue for me has been if it's rude or obnoxious to try and point it out. Today I heard from an agent, an author, and an editor, and this is what I've learned:
You can have faith in the people behind the scenes who are working to make the book the best it can be. They know what they're doing. That said, if you happen to know the author, it's acceptable to let them know you saw something. Between them and their editor, they go over these things a ton, so they'll probably catch it on their own. But if you and the author are homies, it's fine to watch their back. Other than that, it's a no-no.
Could you imagine how overwhelming it would be for every person who got an ARC to point that stuff out? The publishers are much more interested in knowing how you feel about the story, so share that. So there you go! Now read some books and start reviewing!