http://forum.xda-developers.com/showthread.php?t=2125262
http://www.s3forums.com/forum/galaxy-s3-general-discussion/469-white-s3-back-cover-cracked.html
http://forums.androidcentral.com/samsung-galaxy-s3/199849-hairline-crack-appeared-battery-cover.html
and here is a video for ya:
http://www.youtube.com/watch?v=dp6wJfrhDOk
And this is only choosing the top few off the forums. There are at least 3 more per forum.
Well its never happened to me. But then all phones have faults - Look at the iphone 5 and its scratches out of the box and also the bending. Dont try and make out the Galaxy S3 is the only phone to have certain issues.