I guessed what happened the moment I read that you had flown through the
localizer. This is becoming a real problem and I am beginning to think that
there is a fundamental design flaw in advanced avionics systems.

I have noticed a tendency for people to forget to switch on the CDI even
when they are very familiar with the GPS. I have done it myself, seen many
students make the same mistake, and heard from a lot of other pilots that
this is a problem. People are more likely to forget things like this when
they are already stressed. I think this should be emphasized when training
with advanced aircraft systems. Perhaps the task should be automated -- dial
in a localizer or ILS, and the CDI automatically switches to NAV. However, I
can envision problems with this approach as well, so to speak. :-)