Late to the game but have to chime in here.
THIS THIS THIS!!!
I've also heard of people sending the lens to Sigma for adjustment and having it come back in the same or worse condition.
I just got off the phone with buydig to return my THIRD Sigma 50mm 1.4. I'm tired of trying. Odds are that the 4th lens will have the exact same front focusing problem as the last three.
I've come to believe that there is no such thing as a good copy of this lens. If you have the Sigma 50mm 1.4 and think it's tack sharp, it's because you haven't properly tested it for front focusing. There's no way three lenses in a row can have this exact same problem unless the problem is inherent in the design.
I also have the Canon 50mm 1.4 and conducting the exact same test with everything else being equal, the Canon focuses correctly every time. There is just no getting around the fact that the Canon AF works and the Sigma doesn't.
Now, the caveat is that this Sigma front focusing problem is only apparent in certain situations. I'd guess 90% of the time (or more) you will never know there is a problem. But I just don't trust the lens because I have no idea if/when the problem will rear its ugly head.
It pains me to return this lens because it is a superior lens to the Canon in many other ways, and I got it for the ridiculously low price of $359, and that's before whatever CB I got at the time. This lens is a steal at under $359 but sadly I have to send it back for the third time because accurate focusing is more important than anything else when it comes to lenses.
That's exactly why my video showed. The lens in my video was back to Sigma 3 times, the last 2 times with the camera body in the video, to no avail. Canon vs Sigma on the same tripoded camera. Sigma fails.