o got a banana for free the other day as the till was playing up. depending on cost i might not notice, not sure if i'd take it back or night, might depend on where in the month i was....
It depends how honest I think their tills are on a routine basis.
Aldi seem very straight - I've never spotted a mistake in their favour - so I'd tell them when I noticed.
The Co-op are a shambles - the number of twofer/meal/bogof deals that just don't seem to get implemented is scandalous - so I'd keep the occasional error in my favour to compensate for the many ones which favour them.
Tesco are somewhere between the two - so it would depend on my mood (and the value of the item) i guess.
I've reported it to Waitrose on a Home Delivery and was thanked for letting them know but told to keep it. It was about the same price as yours Bazile.
On two occasions after checking my home delivery I have found an item I did not order. Both times I notified the supermarket and both times I was told to keep the item.
Before barcodes fiddling was rife in supermarkets. The assistant would have a small tin of beans (I'm going back to the days when they cost 6d) and ring the price in with every customer. Very few noticed, but if someone did the excuse was, "Oh, is that not yours? Someone must have left it behind". The non-technical term for the money thus raised was 'the bunce'.
On several occasions I have had items in my home delivery that I had not ordered or paid for and I have always told the supermarket. If its perishable I have always been told to keep the item. On the only occasion when it wasn't perishable, it was a whole bag of very expensive cat food. The driver was called and came rushing back for it as it was a large part of someone else's order.