Data General core machines

I worked on mostly Eclipse machines one summer years back. There was one annoying problem with that scheme: the machine's main power switch had three positions, off, on, and locked IIRC, the last being the save to and restore from core. The problem was that this meant that both AC line and a logic signal that ran more or less directly into the middle of one of the two 15" square boards that implemented the CPU were on contacts only a fraction of an inch apart. And every now and then one of those switches would fail in the worst possible way...

It was interesting trying to fix that CPU board set - microcoded diagnostics kept leading us to change more chips (this was gate level TTL, the microcode ROM & RAM and the ALU slices were about the most complex chips), and it always seemed we were making progress. Then they wondered why this one job was taking so long, which is when they told us about this interesting failure mode, and the board set was sent to the scrap heap.