Here's a little background:
Desktop computer came to my desk with 32GB (4x8) DDR3 1600MHz non-ECC, a GTX960 graphics card, and an OS (Windows 7 Pro) that wouldn't boot. After an overnight memory test, and several video memory tests, everything looked great! I repaired the OS offline and booted it up. Black screen and crash. I figured the videocard, which had just had a driver upgrade was faulty, so I popped it out and booted again. Everything looked great, and it ran like a dream. In order to fix the remaining problems, I upgrade to windows 10 per client request. Boots up great after the install. And then... Total annihilation. It rebooted and went straight to chkdsk where it recovered about 40GB of files. WinSxS was gone, the OS was beyond repair, and I was shocked. We tested the RAM for over 18 hours, running 5 passes with memtest, and a pass using another memory testing platform.
Here's the question:
How did all this testing take place without any indication of a problem, before this massive amount of data was nearly destroyed by bad memory? After replacing the memory the system runs fine. What happened, and what kinds of tests will actually catch a problem like this?