I'm running Sierra on a Mac Pro 5,1. I've got 48GB (6x 8GB) of RAM and after updating to the last security update I've got a RAM conflict, I've swapped some modules position and I could get just 32GB working.
I've already faced that when I upgraded to Yosemite before switching to Sierra, but never with security updates.
Any idea on what’s causing that? It seems the Mac OS X doesn’t allow modules from different batches, quite weird that behavior.