At some point hardware devices fail, so every piece of hardware in a control system is a potential point of failure and potential problem. At the lowest control system level we have devices that provide for communication of the monitoring data to the system, with the data usually being a measurement
or state of a device. These are the typical sensors, relays, and transducers. In addition to complete failure of a device (such as a sensor) you can have an operating sensor that's just inaccurate. Sensors need to be recalibrated on a regular basis (many organizations calibrate on a regular schedule.) The issue here is that the control system may be receiving and acting on poor quality data.
At this same control system level are devices that the control system is managing and controlling. These are devices such as valve or damper operators and variable speed drives. Failure of the device, such as a leaking control valve, really negates the control request and overall control strategy of the building system.
The controller themselves may fail. This is typically related to the controller's circuit board, either the components on the circuit board or the board's ability to bond different parts of the board.