We once made an offer mostly because of a question the candidate asked us. We gave them a small task around recording a payment, and before writing a single line they asked what happens if it runs twice.
That instinct is a big part of the job. Real systems retry — a network hiccup, an impatient user, a queue that redelivers — and code written as if that never happens is the code that charges a customer twice. We can teach a framework in a week; the habit of asking what the world does to your work is much harder to install.
What's the best question a candidate has ever asked you in an interview?