One thing that we ran into back in the day was EEC failure on reboot.

We had a few Dell servers that ran great for a year or two. We rebooted one for some reason or another and it refused to POST due to an EEC failure.

Hauled down to the colo at 3AM and ripped the fucking ram out of the box and hoped it would restart.

Hardware fails. The RAM was fine for years, but something happened to it. Even Dell had no idea and just shipped us another stick, which we stuck in at the next downtime window.

To top it off, we dropped the failing RAM into another box at the office and it worked fine. <shrug>.