There was an instance that an installer ran an alarm wire in parallel to a 3phase feed to a refrigeration compressor. Every once in a while the system would give false alarms - especially when the compressor started. Re-routing the alarm cable solved the problem.

If this had been a truly differential driven, received and terminated twisted pair, this would not have happened. So many signals are carried via ground referenced wires (even optocouplers where the supply for the driver is grounded) which make them suspectible to noise pick-up. The best way to defaeat noise is true differential signalling. I stared using RS485 chips for non-serial signalling a while ago and have had zero problems.

Erik