Often a crash is the result of unexpected data. For instance, a crash could be caused by an extra space at the end of a string, or an integer being negative when your code didn't expect it to be. Or maybe the crash happened during the processing of a bunch of files and you want to know the name of the file that caused the crash. What you really want is the ability to inspect the values of arguments and locals at different parts of the stack. But how can we write that data to a log file on a crash without writing it when there is no crash?