Worse is when you think you've proved it to be bad memory, but it turns to be something way more ridiculous. I dealt with a batch of 60 identical PCs that woul

Worse is when you think you've proved it to be bad memory, but it turns to be something way more ridiculous. I dealt with a batch of 60 identical PCs that would occasionally fail in odd ways during the disk imaging process, and upon further inspection would fail memtest spectacularly. It didn't make sense, as they were stable when running an operating system, just not in memtest or while imaging. Plus the memory tested fine in other computers! I eventually figured out that while the BIOS was in real mode, the USB controller would write to extended memory when the USB mouse sent it events. They would have no problems with the mouse unplugged, and the problem went away with a BIOS update. It took a lot of yelling to figure this out!