Walking the Valid State refers to a dilema where a User is forced to modify inputs in a specific order so as to avoid momentarilly entering an invalid state. This usually happens when an interface blocks invalid states in an attemt to simplify the programs. Say a User enters a Coupon into a Pizza Delivery app. The coupon is only valid for three large pizza's, but the User tries to add a fourth. There are many valid outcomes that the User could intend. The User could intend to go back and delete one of their items, or could have desided they'd prefer the Pizza over the discount. At this point, however, they would be forced to go back and remove the offending input first, and then carry on with constructing their order, breaking the flow of their process without much benefit.