Whenever you use the Edit menu options Execute, Display, or Inspect, the message Doit is sent, usually to the object nil. Thus, selecting the message UndefinedObject>>#Doit from the message stack displays, in highlighted text, the expression that triggered the debugger. The message below the lowest ExceptionalEvent>>#signalWith: and above UndefinedObject>>#Doit in the stack usually contains the expression that caused the error.