Passing your driving test in the UK can be a daunting experience for many people and can often lead to disappointment. People want to pass their driving test but are unsure exactly how to do it. The answer may seem obvious - drive safely for 40 minutes - but what is actually expected from the examiner and how much is  'down to luck?' In the UK driving test you are allowed 15 minor faults, but no serious or dangerous faults. So, what is the difference between a minor fault and a serious/dangerous fault? A minor fault is a mistake that has happened that may be a one off. For example if the pupil forgets a mirror check when pulling in/out but there are no other people around and therefore nothing 'serious' is likely to happen. However, if the pupil keeps doing this 3 or 4 times, then the examiner will consider that this is not a 'one off' fault but it is just a matter of time before the pupil's luck runs out and somebody will be there when the pupil pulls in/out. These 3 or 4 'minor' faults are now becoming a serious risk and therefore the pupil will fail as it is considered a serious error and will fail with just 4 minors in the one category. So a person can fail by simply making the same error 3 or 4 times. A serious fault would be a mistake that could have had serious consequences (an accident), but luckily it didn't. An example could be cutting a corner without looking or pulling out at a junction without looking to see it is clear. Even though no accident actually happened, it was pure luck that nothing was coming. As I guess you can imagine, this is a serious problem. Obviously one day the luck of the pupil will run out. A dangerous fault is where an accident was avoided by the examiner or another driver taking action. Using the example above, the pupil pulls out of a junction but does not look and this time something is coming. The examiner or the other driver may brake/swerve to avoid a collision. This is considered dangerous driving, therefore a fail is inevitable.

So, a minor mistake can become serious or even dangerous. Using the not checking a mirror example, as mentioned previously, making the same mistake can be considered serious, but also not checking the mirror and someone else is there then the examiner may have to brake to stop a collision, or grab the wheel.
The best thing to do before your test is to get your driving instructor to conduct a mock driving test and aim to make no mistakes. If you fail this, then maybe it would be wise to move your test, because once you add nerves into the test it will feel much harder.
Good luck if you are about to sit your test and I hope this has helped you understand the marking process.