Sigma has issued a Service Notice
regarding the following lenses and serial number ranges:
APO 50-500mm F4.5-6.3 DG OS HSM (10633051 - 10972000)
APO 120-400mm F4.5-5.6 DG OS HSM (10574001 - 10972000)
APO 150-500mm F5-6.3 DG OS HSM (10674301 - 10972000)
Details of the issue being resolved are vague: "possible autofocus defect".