One of the first things my dad taught me (he's a computer scientist) is to develop and use error-handling routines. No user of your system wants to see error messages that mean nothing, or to have no error messages show up at all when a process fails. If you can do a walkthrough of your code and look closely at the output it provides to the end user, you can usually do a better job of predicting where people will need support or have complaints about their confusion.