Kathleen likes avocado and crab dip. After eating avocado and crab dip with four crackers, she switches to cheese with crackers. We can conclude that:
the avocado and crab dip cannot have tasted that good.
the avocado and crab dip with crackers now has a marginal utility of zero.
at this point cheese and crackers have a higher marginal utility per dollar spent than that of avocado and crab dip with crackers.
Kathleen is no longer maximizing her utility.