This is more a word of warning rather than a PQ issue. There are times when a piece of equipment malfunctions due to an external influence that is not mains borne. Sometimes it is simply the proximity of incompatible devices to each other.
A prime example is welding machines. These use high current. High current causes high magnetic fields. The cables are usually not run together so there is no cancellation. If this is strong enough it can be found to produce voltages on the circuitry in the second device which could be a sensitive microprocessor controlled instrument. The processor asks itself "what the heck is this?", and....... the rest is history - as they say.
Another prime example is plastic welding machines. Some use a very high frequency (a number of MHz at many kW) produced across a set of exposed electrodes. Although the circuit is tuned and most of the energy remains within the vicinity of the welder, just a few watts escaping into the surrounds and then getting picked up on a nearby comms cable can play havoc with the pieces of equipment on the end of this cable.
When the complaint is "when I operate x, y goes stupid" then look at things like proximity, shared power points, etc.