In a nice Speech Technology mag article from Dr. Susan, she highlights that people, in this case callers to automated systems, frequently behave like water (my interpretation, not hers). That is, they try to find the easiest way to get what they want most of the time. Other times, they barrel over any seemingly arbitrary obstacles when they perceive that there is a way to accomplish what they want.
She rightly points out the fine details of how to deal with these issues in a couple of specific situations. The larger lessons are: a) if you make rules (right or wrong) for customers, make sure they know them and make sure they are consistent across the board; b) to maximize success all the way around, facilitate the fastest, easiest way (according to the customer!) to get where the person wants to go.
Too often, our designs provide interaction that is more like dams and channel locks and less like soothing mountain streams.