Make sure that users never lose their work as a result of error, be it on their part, or the fault of the system.
For example, let's say a user fills up a shopping cart with products. Then, accidentally, their browser is closed, or it crashes. When they return to your site, the shopping cart should still contain the items they had previously added.
Nearly all users would be most frustrated to find an empty shopping cart. Many users probably couldn't be bothered to find and add the items to the user's shopping cart again, resulting in a lost sale for you.