When running vncviewer from an icon on the desktop, sometimes
authentication is refused even though the correct password is entered into
the dialog box.

This seems to be because of constructs such as this:

password = XtNewString(XawDialogGetValueString(dialog));

XawDialogGetValueString frees and reallocates a value to return each time,
and XtNewString is a macro which uses its argument many times, and so if
XawDialogGVS returns a different value on successful calls during the
execution of the expansion of the XtNewString macro, "odd" things happen,
and password ends up pointing to the wrong place.