The short answer
The set manoeuvres rarely fail on steering technique. The typical causes are: too fast, too little all-round observation during the manoeuvre, and finishing the task before the end — eyes forward before the car is stationary. Correcting is allowed; abandoning a manoeuvre without securing the car is not.
Which manoeuvres exist is already set out elsewhere. This is about the more interesting question: why they go wrong when they worked twenty times on the practice ground.
Cause one: speed
Almost every manoeuvre is driven faster in the test than in training, because tension makes the foot lighter and because nobody wants to keep the examiner waiting. Walking pace is the right speed for every reversing task — and it is slower than it feels. With time you can correct; without it you have to guess.
Cause two: the eyes stay in the mirror
When reversing, your eyes belong behind you and all around, not on a reflection. The mirror shows you the kerb; it does not show you the child stepping out between two cars. In a reversing task, awareness is assessed just as heavily as the result.
Cause three: finished too early
The task is over when the car is stationary and secured. Rolling to a halt while already looking forward is the moment in which most kerb contacts happen — after fifteen correct seconds.
"Just stop for a moment." Not as a correction, but because stopping halfway through a manoeuvre is allowed and almost nobody thinks of it. If you notice while parking that the angle is wrong, you may halt, look, and start again. That costs nothing. Carrying on shunting in the hope that it will sort itself out costs the task.
What is allowed
- Correcting: a second attempt while parking is not a mistake
- Stopping and starting again, as long as the car is secured
- A failed task may as a rule be repeated once
- Opening the window or turning round as far as the belt allows
What practice grounds leave out
There are no cars with people in them on a practice ground, and nobody is waiting behind you. So practise deliberately where there is traffic — a real parking space in Flingern or Bilk in the evening, with everything uncomfortable about it. That is the difference between the manoeuvre and the situation.
What ends the drive immediately, by contrast, is here.
What to take away
Reversing fails on speed and observation, never on the steering wheel. If you change one thing in the test compared with training, change this: slower, and with your head actually turned round.