Brant, you were right -- something was off. I did a bit more testing this morning. I believe the errors I was getting were the result of the aperture...
Not long ago I bought a used Canon EF 400mm f/2.8L IS II in great condition off of eBay (it was pristine). For kicks, I took it outside to the street...
Yep. It's running the most recent firmware. It's impossible to shoot it at anything but f/1.4 and locks up the camera if you try to change apertures...
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. john 3:16