I unplugged the wrong power cable once, and thereafter the RAID controller on that machine would report degradation after the system was running for a while. Finally ran a RAM error check, one bit in most of a stick was flipped. The last stick. So once the machine was all spooled up for a while, I’d get errors.

This is also how I learned 3ware was using RAM instead of its own memory. What is he point of hardware RAID if main memory can screw it up?

I have also heard of people swapping hardware on a defective machine only to learn that the power supply was bad, sending noise or brown power to the system. Swap in a new power supply and the components start to behave themselves.