I usually forget to buy something while grocery shopping, whether it be in person, or ordering. There’s no pattern behind it, the items are completely unrelated to a theme (eg. coffee, soap, mouthwash, bread, milk, eggs, potatoes, salt, etc.) as far as I can tell, I just forget to buy something.
Oh, I type out a full list every time on my phone, check it about 3-4 times when starting, then I get cocky as “of course I remember what’s on the list, I read it 5 minutes ago…” and proceed to forget about that one thing.