Sun E250
I had a Sun E250 around the year 1999 or so that every two weeks, give or take a few days, would develop a fault on one of the CPUs and crash. Sun were dutifully called out (except it wasn't Sun that came, it was some outsourcing company) and replaced the CPU. The first time I remember the guy opened the box up and looked on the motherboard for the label for which CPU had the fault and couldn't see anything. I had to point out the mobo layout was on the underside of the access panel he had removed
After 3 or 4 CPU replacements someone at Sun started wanting to figure out what as killing CPUs. PSUs were replaced, then the motherboard and still the CPU killer was there. Seems that since the E250 had dual PSU, there was a power distribution board (probably with PMBUS functionality to monitor the PSUs also) hidden under the mobo. Swapping that finally sorted the CPU killer
Never figured out why it was always killing one of the dual CPUs in the chassis