A while back due to a freak accident, the glass broke on my S3. I decided to buy a kit and replace it myself. It went o.k. but I wasn't too crazy about the result. Touch didn't work as well afterwords and the home button was a little too recessed. I figured either I didn't get the adhesive set right or the glass was thicker.
Not long after I fixed it ( within 6 months?) my daughter knocked my phone off a counter and the glass broke again. So I ordered another kit.
This one came with a sticker on the glass packaging. It said, "There is a thin layer of plastic on the glass that is very difficult to see. Be sure to remove it."
As I pulled up the glass I installed the first time it broke - it pealed away with a layer of plastic under it. The first kit hadn't had that warning and I installed the glass without removing the plastic. Now that I have it in correctly it fits much better and everything works much better. I celebrated by updating Cyanogenmod and now I'm running KitKat.
So my daughter did me a favor busting the glass on my phone.