UndoManager by default undoes file operations if an uncaught exception occurs. This is useful for production projects but inconvenient when in development mode as it makes it unduly difficult to determine the source of an uncaught exception if that source originated from a temporary on-disk file (as is so often the case with parsing source files). As such UndoManager should not undo if the user is in development mode, thus leaving potentially corrupt files in-place for subsequent examination.