Howdy!
Yes, I've seen an IFR GPS disagree with a VFR LORAN, a handheld GPS, and the view out the window. The IFR GPS was the most correct, and the view out the window was the absolute correct.
I've watched another IFR capable GPS, VFR installation, quit with database and other errors while the VOR continued working.
I can tell when a student is using GPS navigation versus the VOR because the VOR scallops on a certain approach. Lotsa fun.
I've watched a student fly a GPS approach in actual to see the "RAIM not available" message just before the FAF. It gave the student a much needed wake-up call. If he would have checked the GPS NOTAMS, he would have flown the VOR approach instead.
I watched another GPS unit, IFR installation, just not certified, give an "estimated position" when it lost signal reception from all but two satellites. The NDB was the only signal available in that particular situation. There was a lot of rock between us and the other satellites.
I've also seen all sorts of wierd and unusual error messages from the units when installed incorrectly. Usually it is a missing ground wire connection, or two flip-flopped. The worst occur when the shop forgets to reset the sensitivity, squelch, and OBS setting.
As for the eBay sale on both the KLN89B and Garmin 430, I know the manufacturers tend to yank dealership authorizations for selling new units there, used, well, good luck. Buyer beware. It will be interesting to see how much those go for. The completed sales on the GARMIN 430s show about $5200 through $5900. Not much more money and one gets a new unit; $960 through 1800 on the KLN series. Still much cheaper than a couple of pilot certificates.
Fly SAFE!
Jedi Nein