We've all experienced it. Someone has green stuff in her teeth, or someone else has buttoned his sweater wrong, or someone consistenly mispronounces "nuclear". Do you tell the person about it?
Some of us would. Some wouldn't. Some feel it's their duty to help or believe it's a kindness to spare another person further embarrassment. And some just like finding fault.
Authors get things wrong, and readers notice. The question is tough for said reader: does he tell the author, or let her wander along in ignorant bliss?
Good question. I have no answer, but here are a few considerations.
If you happen to be an expert on a particular subject, you probably know things that most readers won't. Does it matter if the murder weapon is wrong for that decade?
How many others have already pointed the mistake out to the author? If you're the twentieth or the fortieth, the author may have trouble being gracious.
On the other hand, most authors want to know what's wrong with their work. Not that it can be changed, but future errors can be avoided once we're aware of them. No one is a 100 percent expert on everything, and it's easy to slide into error with a casual reference, example, or idiom.
So when you notice that "something" on my teeth or in my book, consider whether it's to my benefit to point it out. If you're being helpful, I'll be thankful. If you're being picky, I'd prefer you pick on someon else.