I did real life grocery shopping today (get it delivered 95% of the time) and went prepared with a bottle, water and snack to keep DS happy.
While I was in the biscuit isle, i went past what looked like 2 kids in a trolley, their mum and their grandmother. The grandmother picks some packs of biscuits off the shelves, opens one of the packets and gives each of the girls a cookie saying something like "this should keep you happy/quiet".
Personally I think it's quite rude to do that - open something before you've paid for it, even if you intend on paying. Am I alone?!