The Problem with "Perspective Readers"

When I was fifteen, I proofread a book for an Australian Jewish adult male writer. I was a Utah Mormon teen girl, just like his protagonist. As well as giving him general input on the story I was supposed to tell him how well he portrayed her. So I read the book, gave some feedback, put it on a shelf, and didn't think about it again for three years.
This week I found the book again while cleaning out my room. There’s a scene where the protagonist, who had newly arrived in Australia, was confused when she heard the Australians refer to a barbecue as “the barbie”. That’s Australian slang, definitely, but it’s a piece of Australian slang all Americans know. She shouldn’t have been confused. After reading 300 pages of this book I was more concerned about plot structure and disappearing characters to catch something so small. I never told the author about the barbie issue.

The second most consistent piece of advice I hear about writing “the other” is to find someone who fits in the same boxes as their protagonist to read over it. This isn’t always helpful. No one reader can catch every problem and not every reader will view the same things as problems.