Cody recently relayed a Gizmodo report describing how an unknown portion of iPhone 5s owners have been experiencing inaccurate compass and inclinometers readings, consistently off by a noticeable margin. A huge thread on Apple's support forums details the persistent problem.
One report Wednesday offered the most likely culprit: a change in supplier. It wasn't immediately clear whether the issue can be fixed by way of firmware update or if some sensors are off by design, in which case Apple might replace faulty iPhone 5s units at no cost...