I didn't know about the "if it's mischarged it's free" policy. My local supermarket (not Coles or Woolies) always rings things in at the till wrong and I feel like I'm watching them like a hawk.
I used to do it all the time - it's great when they stuff up prices on expensive items like Razors. One time I went back every day and got a free pack and ended up with several years worth of (free) expensive razors in surplus!
Usually the manager needs to come because the basic checkout person doesn't know about it.