Something that's always correct or accurate is unerring. You can count on your best friend's unerring sense of style whenever you're trying to decide which shoes go with your outfit.
To err is to do something wrong or make an error, so unerring means never making mistakes or missteps. Your unerring sense of direction means your family relies on you to lead the way in unfamiliar cities. The unerring accuracy of your mom's pitch makes her a superstar on her softball team. And your brother's unerring logic makes him an extremely frustrating person to argue with.