When we do our main shop we always take a shopping list. There are things that we always get that we never put on the list though - like crisps, chocolate, fish, and prawns. My dad is also notorious for buying lots of cheap meat that is on offer . It annoys the heck out of my mum because she then needs to find a place for it in the freezer, when there isn't any.
On smaller shops we don't make a list because know the things that we are going for specifically for. We always end up getting a lot more than intended though.
Occasionally we end up not get getting things that were on the list too. I sometimes miss them by mistake, forget, or can't read my mum's handwriting!