First I have to say that I’m Sports photographer. (Soccer)
I tried my new Sigma 70-200 F2.8 lense last weekend and saw that most of the photos (8/10) were "back-focus". I had no problems with other lenses. At home, I tried more shootings and saw the same problem. But when I turned OFF my camera, changed the lense and put it back on again, it worked OK.
I think the problem is caused when I put the lense on while the camera was ON.
Is any one out there experiencing the same problem?

Thank you for your time.
Harry

It's always best to change lenses after turning the camera off because you'll cross the contacts between the lens and the body when you twist it off (not sure it's a problem, but it can't help).

A good check for accurate focusing is to use the lens at 200mm and f2.8, then focus on something at about 10' or less distance. Use a tripod to rule out any camera shake, and be very careful with the focus. Since you've got very little depth of field, it will be very obvious if it's right or not.