I've had a couple of issues recently with unexplained customer systems failures. We've had one system that was intermittently dropping its RS485 comms network and on a completely different installation we've had 2 failures of an actual PLC within 2 months.
With the comms failures we went through the usual proceedures of checking the integrety of the power, the earthing, bonding and network shielding arrangements as well as checking the clamping voltages and effective operation of the surge arrestors that were installed. Nothing came back as problematic but 3 network cards later I put a power quality analyzer on the system for a 5 day soak test. The first and most glaring problem that showed up was that the supply voltage after load shedding incidents was as low as 176V immediately after it was restored. 15 minutes after the power was restored it was still hovering around the 195 volt mark and it was a full hour and a half before it was stable at around 118v and over 2 1/2 hours before it approached the 'normal' supply voltage for that particular installation which is 238v (they are the closest customer to the Eskom MV transformer).
I installed the same PQA on the PLC control panel at a different customer and it also showed a similar low voltage issue for an equally long period after a load shedding incident. Both of these systems have now had their power supply units replaced with alternatives that have a wider tolerances of input voltage and, touch wood, both issues seem to have been resolved.