Open loop vs closed loop in control theory
You can't implement a stable closed-loop system without knowing what the open-loop response looks like. 

A simple example might be controlling the brightness of an incandescent lamp using a photo-diode (very fast) as feedback of brightness.

At rest, the system has no-problem then you set a demand that wants to see X watts per square metre produced at 1m. The photodiode will tell you the watts per square metre hitting it but if you don't take into account the time-lag (or inertia) of the lamp your control system will ramp up to maximum power before your lamp has started to glow.

The photo-diode, at some point later registers the correct amount of light and the driving system instantly "levels-out" because it believes the lamp has hit the demand but, the lamp will glow a bit brighter because of thermal lag (or inertia) and then the control loop will switch off and what you'll get is possibly a self-oscillating system and it may take ages before the system settles down.

Along the way you may even destroy the lamp.

What about other control loops for things like linear actuators - you set a demand position and an amplifier starts driving the motor to the correct position but you get overshoot because the motor and mechanism have inertia.

Basically, if you don't respect the open-loop response you have a recipe for disaster.