While in produce he'll see the guacamole. Seeing the guacamole, he will think of chips.
When he sees the chips, they will remind him of white salsa con queso dip. He'll probably grab some.
After wandering to the chip aisle, he'll spot some Oreos..."For the girls," he'll justify.
Being halfway into the store, he will figure he might as well go all the way to the back to get some yogurt.
On the way, he'll see some bay leaves...and some fennel seeds too.
When he picks up the fennel, he'll see a crazy Wal-Mart shopper that he should take a picture of.
Thinking of pictures will remind him of his original assignment.
And chances are, when he leaves the stores, he'll won't get celery, which we actually needed.