So far we've been working with examples which work the first time. If you didn't type them in correctly, you probably received a flood of unintelligible complaints. You probably ignored the complaints, and typed the example again.
When developing your own Smalltalk code, however, these messages are the way you find out what went wrong. Because your objects, their methods, the error printout, and your interactive environment are all contained within the same Smalltalk session, you can use these error messages to debug your code using very powerful techniques.