I stopped flying the classic in '98, but I DO remember thinking there was a weakness in the instrumentation. When engaged in a navigation mode (vor, ins, etc) a white flag showed 'nav'. However, when in heading mode, there was NO flag showing...now, on a crosscheck, human nature looks for discrepencies. A white 'nav' flag when wanting 'heading' mode, and vice versa, is quite easy to spot. However, the LACK of a flag is like a passive warning...I caught myself, and first officers, more than once because of this (albeit never more than a few min). Why didn't Boeing put a white 'Heading' mode flag showing positive engagement of heading? A little trap, and may have cost many lives.