Integral windup

Every real actuator has limits: a valve can only open fully, a heater has a maximum power, a motor driver has a current limit. If the controller asks for more than the actuator can deliver for a long time, the integral term keeps growing even though nothing more is happening physically. When conditions change, that stored-up integral has to unwind before the output comes off the limit, producing a large overshoot or a long stall.

Typical triggers are startup from a cold or extreme state, large setpoint changes, a door left open, or a jammed mechanism. The fix is anti-windup logic: stop or clamp integration while the output is saturated, or back-calculate the integrator from the actual actuator output. Most PID implementations offer this, but it has to be enabled and configured for each loop.