My HTC One X on AT&T had a similar issue. It had <20% battery life remaining when it crashed, but still was above 10% for sure. I went to unlock the phone a few minutes later and it would not turn on. Held the power button, nothing. Plugged it into the charger, nothing. Not even a charging LED. After 5 minutes the charging LED came on intermittently but kept going away. Holding the power button caused the capacitive touch buttons to blink rapidly and still wouldn't turn on. After 15 minutes the charging LED came back on solid red. Then when I held the power button the phone turned back on. I've been using it for about a week since then. When I first got the phone, I was getting AMAZINGLY PHENOMENAL battery life. Ever since this incident, I have been getting TERRIBLE battery life. Like 1/3 the battery life. And I checked booting it up holding the volume down key and there is no option to delete or reset the battery calibration. I have already let the battery discharge down to 0 and recharged it, still abysmal battery life compared to the first week. I am considering exchanging it for another at the AT&T store as it's the BEST device I've ever used, but not sure if AT&T will simply exchange it due to me claiming it all of a sudden has much worse battery life since there is nothing I can really demonstrate in the store to the sales reps. I am a new AT&T customer that just switched to the carrier and I'm not sure if exchanging a device there is as ridiculously painful as it used to be at Sprint stores...