First off, this whole debate is theoretical since I am male. I don't buy groceries on a frequent basis (once every two to three weeks) so it isn't like I'm there a lot. I am also quite of a space case and don't focus on the present but am thinking about other things, which means I forget things while grocery shopping quite frequently. There are some times that it would be just walking over and adding it to the cart, but for me personally, I'm sure I would forget more often than not. (Of course, that is easy to say as a guy who doesn't have to deal with menstruation)