From memory it was something to do with the focussing mechanism not working correctly. There does sometimes appear to be a quality control issue with Sigma. TBH the 100-300 is a bit heavy to cart around but I recently got another 50-150 f2.8 which is a great walkaround lens and has the advantage of being good for available light indoor stuff. Really meant to get a 70-200 f2.8 but just thought I'd put a bid on a bargain 100-300 ...expecting to be outbid....took a look in the morning and found I'd won it!