I was thinking about the idea of lost purposes in my kitchen, and a vivid illustration of the idea occurred to me:

You plan to make homemade ice cream for your partner's birthday party next week, so you put "cream" on your shopping list. The next day, you break up with your partner on surprisingly unfriendly terms. You are no longer going to be attending the birthday party. But then you find yourself at the supermarket, with your shopping list in hand, putting a carton of cream into your cart.

EDIT: The birthday-party/break-up thing is a fictional scenario, not something that actually happened to me. Sorry for any worries!