It helps to be prepared for what might happen. If you have an umbrella when it starts to rain, then you’ll be much drier and happier than someone without an umbrella. On the web, it helps to consider all of the possibilities.
Consider these cases: none, one, some, and lots. For example, a search results page might have a special message when zero results are found, and there might be a different message for one and two results because 1 is singular (“1 result found”) and 2 is plural (“2 results found”). There might not be any difference between 2 and 3 results because they are both plural, but if 100 results are found, then you’ll probably want to break up the results into several pages.
Consider the environment in which your product will be used. In his book, The Design of Everyday Things, Donald Norman points out that many car radios are easy to operate if the car is parked but MUCH harder operate while driving (because the buttons and dials are so small), and most car radios are operated by someone who is driving.
Consider extreme cases. Most fire extinguishers will never be used, and that’s a good thing but it’s also a good thing that they are there, in case there’s ever a fire. They are there because someone realized that under extreme and dire circumstances, they would be necessary. When designing a website, it’s necessary to consider all of the possible actions that a user might perform and also the different sequences in which all of those possible actions might be performed.