This video shows the KD01 (differential drive) robot executing a simple rotate-in-place movement using its PID controllers to accelerate to a set velocity, decelerate to zero, and then spin in reverse.
As electric motors don't generally run the same speed in both directions the PID controllers should compensate for this, and to a reasonable extent this test shows that they do, as the robot remains largely in the same location at the end of the test. Not perfect, but close.
The same test done with no PID controllers (direct control of the motor speed) has the robot slowly pirouetting slowly across the floor, ending a few inches from the starting point. This error is evidence that even with the PID controllers the mechanics of the KD01 design are not perfect, i.e., that the alignment of the motors, location of the caster, weight balance or some other factors are at play.