Customers judge our service by whether we meet, exceed, or fall short of their expectations. When we get a chance to influence a customer's expectations, we should be careful to set ourselves up for success.

Example:
Let's say a customer asks you to do something that takes 30 minutes for you to do. You probably tell your customer you'll get it done in 30 minutes, right?  

That's a very dangerous promise. What if something unexpected comes up and it takes you longer than 30 minutes to complete? In the eyes of the customer, you've taken longer than you promised.

A better approach is to set expectations that provide you with some wiggle room while still being acceptable to your customer.

If the task will take you 30 minutes, why not see if the customer will agree to a one hour turnaround time? Getting back to the customer within 30 minutes will exceed their expectations. If something comes up and it takes you an hour, you've simply met your customer's expectations, and nobody is upset.