From another forum and DPR - Some Sigma lens are experiencing some focusing problems.
They have listed the following -
50-500 f4.5-6.3 DG OS HSM serials 10633051-10972000
120-400 f4.5-5.6 DG OS HSM serials 10574001-10972000
150-500 f5-6.3 DG OS HSM serials 10674301-10972000
Apparently only the lens within the serial ranges are affected.
They can be returned to Sigma agents for checking.