Most errors in Smalltalk are caused by messages sent to objects which do not understand them. Smalltalk does not check to see if an object can handle a message until run-time.
Run visualworks, and file in the DebugLab.st source file for programming assignment 1.
Look at the Account and AccountList classes in the new DebugLab category. If you can't find that category, update the System code browser by selecting update in the category list menu (click right mouse button in the category pane).
Enter the following code in the workspace, select it and run it (choose do it from the right button menu):
|a|
a := Account new initialize: 102054 name: 'John Brown'.
a monthlyOperations
An exception window appears.