I've had this happen, although not at the 40% like they claim. Mine frequently happened around 20%, and over time increasingly around 30%. By that time I swapped it out with Apple for a replacement due to that issue, plus two other unrelated defects. A friend had this happening to him too, he got a free replacement phone from Apple. To me, that suggests Apple is aware of the issue. It's a unfortunate bug to have. If you don't have a charging cord/device near by, and are stranded on a deserted road with a phone which has the power, but doesn't think it has the power, you could be SOL.