I usually remember what I actually went in for when I am just reaching a till. Then it is a toss up whether to put groceries on belt and run back, or just go away back to shopping. The third option is one I have done successfully and throw myself on the mercy of the till girl. I have just remembered I need X. And she finds someone to go and get it.
Jeza you are perfectly normal. We all do it all the time.
I always do a big shop on Mondays coz it's quieter, but every Monday even when the 'fridge is packed I still have nothing for the evening meal and end up with egg and chips.