As a former Liquorland and Coles employee I thought I'd shed to light some information that is seemingly not well known from what I've seen reading other threads.
Firstly, Coles and Woolworths are signatories of a little thing called the scanning code of practice. Whilst this code is voluntary, the supermarkets DO follow it (see: http://www.anra.com.au/Scanning%20Code%20of%20Practice%20for…).
Essentially what this means, is that if your item scans at a higher price than the ticket price, you get it for free (it only applies to the first item that scans wrong in each transaction though). So, if you're looking for free food or any other supermarket good, make sure you note the price before heading to the checkout. The easiest way to get free food is benefiting from lazy staff who haven't taken the previous weeks 'specials' tickets down. Wednesdays are best for this, as it is the first day of new specials. I've gotten countless free products from doing this and you are quite within your rights to ask for it too, even if the staff seem begrudging they should be well aware of this or will find someone that does know about it. There is even a button on their screen for free items due to scanning errors.
As a previous Liquorland employee this code is NOT applicable to liquor or tobacco purchases as some customers have told me, BUT Liquorland does have its own scanning practice. If your item scans at a higher price than the ticket price, you are entitled to that item at the ticket press LESS a 10% discount. Staff will usually try and give it to you at the ticket price and you may feel satisfied with this but also note that you should push for them to give you the discount as well. In my time there only one person was ever aware that this was the practice. Obviously this is dependant on staff missing specials tickets, or prices being incorrectly set so it involves an element of luck and dependence on lazy staff. In my experience, the smaller stores that get less traffic are more likely to be slow changing tickets.
Hope this has helped some people and provided a bit of clarification :)
Thanks for this mate.