I think he's talking about shifting it one gear at a time like it's a 3-on-the-tree. The problem is that you can slip a shift and either put it in neutral, or in reverse. I've never had your situation happen, but I did accidentally put it in reverse while moving forward when I was testing the adjustment on my new floor shifter. It definately made some nasty noises, but I was able to pull it back into neutral before anything happened, and I was only going about 15 mph when I did it. I've also done what you're talking about while testdriving a new 07 with the floor shifter, but that is a really soft shifter, and it's also really easy to slip shifts.
That's actually how the slap-shift function works on my floor shifter, the mechanism advances one gear from 1st to 2nd to drive, but the differance is it's designed to not allow it to advance any further than one gear at a time, and it won't advance past drive until you put it back into normal gate mode.
With yours, the only thing I can figure is for whatever reason, either the gear shifter didn't fully engage the shift switch on the side of the transmission into reverse, or you've got an internal issue, but I don't know.