Great question! As someone who's worked with industrial automation, I'd say the three biggest PLC-to-robot communication nightmares in a lights-out factory would be:

1. **Network Communication Breakdowns** - When you're running 24/7 with no humans on-site, network failures become catastrophic. Industrial protocols like EtherNet/IP, Profinet, or Modbus TCP can fail due to electrical noise interference, network congestion from adding new devices without proper planning, or simple cable/connector degradation over time. The worst part? These failures often happen intermittently, making them incredibly hard to diagnose remotely.

2. **Protocol Mismatch and Timing Issues** - Different PLC and robot brands often speak slightly different 'dialects' of the same protocol. Even tiny timing mismatches can cause robots to miss critical commands or receive corrupted data. In a lights-out environment, you can't have a technician physically checking handshake protocols or resetting communication buffers when things get out of sync.

3. **Power Supply and Electrical Noise Interference** - This is the silent killer. Power fluctuations or electrical noise can corrupt PLC command codes mid-transmission, causing robots to execute wrong movements or stop unexpectedly. In a fully automated factory, you're dealing with massive motors, welders, and other equipment that generate significant electrical noise that can disrupt delicate communication signals.

What makes these especially scary in a lights-out factory is that they're often subtle, intermittent failures that require physical inspection to diagnose - exactly what you don't have when there's no one on the floor. You'd need robust monitoring, redundant systems, and predictive maintenance to sleep soundly!