iPhone 5S Users Reporting Blue Screen of Death
The Numbers app in Apple's iWork suite, a free program with new iPhones, seems to be the primary cause, though BSoD behavior has also been observed in other applications, according to complaints in Apple's support forum. Here's a demonstration of the error posted to YouTube.
Based on when the support threads were first posted, this appears to be a problem that's been going on since the iPhone 5S launched last month, though it's just now gaining attention. Apple hasn't acknowledged the problem, so it's unclear when a fix might be in sight. Some users say that disabling iCloud syncing to the iWork suite seems to work for some, though not every user.