At one time or another, most applications have to determine when a certain subroutine or function is executed and inform the user when an important event or an error occurred. Over the years, clever developers created different methods for these kinds of notifications. At some point in time, just about every Visual Basic programmer has tried using a message box to show when a method or subroutine was called. While this technique might work well for debugging, it's highly error-prone. Because there are legitimate times when a message box should be shown to the user, the developer can't just comment out every call to MessageBox.