Also keep in mind that it’s not just RAM that can have issues, but also the slots it sits in.

I once had a server-grade workstation (dual-socket AMD Athlon MP, 8 slots with 4Gb ECC REG apiece). Each piece of RAM tested perfectly OK by itself in the default primary slot, but failed consistently in one secondary slot and intermittently in another. The slot hardware (pins) were fine, but something elsewhere in the motherboard had broke.

Which is why you also test each slot with a known good stick of RAM, just to be absolutely sure.