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It's called the Supermarket Scanning Code of Conduct. If it scans wrong, you get the first one free, and the rest at the proper price. It's designed to keep the ACCC off their back.

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.