So, I decided to max the RAM out on my #Lenovo #Thinkstation #P340. It takes 128GB of RAM. It gets here, I install it, machine just turns off and on. I troubleshoot, and if I pull the ram from DIMM slot 4, it boots fine. Not a bad stick as I rotate through them and they all test fine. I reinstall the original ram (2x 16GB sticks) and I notice the same slot is bad.
Call in to get warranty work, get the mobo swapped, same issue.
While the tech is on the phone to get another mobo on the way, I get some idea that maybe it's an allocation issue since it's happening during POST- you know- like the old #BeOS >1GB RAM issue. I pull my 3.2TB Samsung PCIe AIC NVME and
, machine boots and posts all ram- including DIMM slot 4. I try my NVME in another slot and same issue. 
So, weird issue- I can't have a PCIe AIC NVME (or at least this one anyway) and have all 4 ram slots work in a Lenovo ThinkStation P340 tower system. You'd think for a workstation-class machine, this wouldn't be an issue. But here we are. 128GB of RAM or an additional 3.2TB of screamin fast PCIe NVME server-class storage. 