Reversing manoeuvres require very low speed, accurate steering and continuous observation.
You should make effective all-round observations, show consideration to other road users and keep reasonable control of distance from the kerb.
These examples are for understanding and practice; they are not a promise that a particular mistake will always receive a particular grade.
Watching mainly one mirror and failing to notice a pedestrian or vehicle.
Going too fast, mounting the kerb or swinging excessively wide.