We must keep in mind that there is more to the "System" than just the furnace and air conditioner. The definition of a system is: A set of things working together as parts of a mechanism or an interconnecting network.  

When ANY part of the system fails, the entire network also fails. HVAC is by no means exempt. When we have equipment or component failure, we are quick to assume that the equipment is bad, or the factory did a bad job during testing, or quality checks. We sometimes refer to this as a "Monday" part. Although this can be true in some cases, we should do our job as thorough and diligent technicians, and evaluate the "System". 

In HVAC we have other systems connected to our equipment that must be in good working order, so the ENTIRE System will operate correctly and efficiently. 

Duct system, with out it, we cannot move or transfer heat. If the duct system is not built correctly (or if the wrong size equipment is attached) we can overheat, freeze up, short-cycle, ruin a compressor, crack a heat-exchanger, produce loud sounds and lastly, upset our customer. 

Electrical system, power no good, furnace/ac no good. Too much power, not enough power, short circuits, bad/dirty grounds, shared circuits can all lead to sub-par operation of the equipment and possibly board or component failure.

Vent system (chimney), if the vent system is not done correctly, or is plugged/blocked/restricted, then our furnace/ac will not work correctly either.

Condensate system, if we are producing condensate, then we need to get rid of it. If the condensate system fails, we can have a huge mess or a piece of equipment that won't run.

Gas system, combustible and refrigerant gas. If that system is not built correctly or has an issue, we lose performance or may not run at all.

The building itself, if the box we are putting all of this in, has issues with it, such as negative pressure, poor air quality or high rates of infiltration, then the equipment will respond negatively. 

Keep an open eye and an open mind when installing or troubleshooting, the route cause to the failure may not always be an issue with the equipment, but something  else connected to the system.