I had an interesting experience earlier today. I thought after all this
time I had the CNX-80 figured out, but I found a new way to screw things up.

I was on the POU GPS-24 (http://204.108.4.16/d-tpp/0508/00286VDG24.PDF),
full procedure, Kingston IAF. We were IFR, in and out of IMC. We had been
cleared for the appoach, which was loaded and executed in the GPS. We were
on a heading of roughly 180 to IGN, and about a mile from the VOR, when the
controller said something like, "I need to you to stay at 4000 and keep
tracking outbound for a while for traffic below you". I agreed to do so
and turned to a 060 heading.

At this point, things started going downhill. I suppose, technically the
controller committed an error. He had already cleared me for the approach,
but then gave me instructions contrary to the procedure. I was in a sort
of half-way place between flying the full procedure pilot nav and being on
vectors. Better to have just canceled my approach clearance and given me
vectors to avoid the traffic then back around for a fresh start.

I was equally at fault for accepting this. But, now that I had made my
pact with the devil, I needed to convince the GPS to play along with our
game. The box hadn't yet sequenced past IGN, when I started my turn
outbound. This seemed to have confused the computer. We were heading
outbound on the PT, but the GPS was still navigating to IGN. In an attempt
to convince it otherwise, I pulled up the flight plan page and selected
direct to CFGUY, which got me what looked like positive (and correct)
course guidance on the outbound leg.

Eventually the controller told me I could turn back inbound and descend to
2900. I did so and found myself tracking inbound, but with the CDI giving
me indications opposite of what it should have been, apparently the box
thinking I was still trying to track outbound. I could see (from the
moving map) that I was south of the final approach course, but the CDI was
giving me "fly left" indication. At some point, while we were puzzling
over this, the CDI suddenly flipped to "fly right"; I think this might have
been as we passed CFGUY inbound. Everything proceeded normally from that
point.

I think the moral of the story here is that the computer is not good at
ad-libbing procedures. Accepting an early turn outbound and an extended PT
would have been a no-brainer with a real VOR-DME setup. With the GPS,
since that wasn't what it was programmed to expect, things got ugly. On
the other hand, with the moving map, it's hard to go too far wrong. As
long as I had a picture of where I was relative to IGN, I could pretty much
fly the desired track by eye as I sorted things out.