Most probably the common points of failures are Not taking proper observation before emerging at junctions and Speed on approach these two separate faults often occur at the same time and either one could result in Failure. Suppose if you have a cross by the side of not taking proper observation before emerging at a junction what does it mean? It means that you have driven onto a roundabout or emerged at a junction impeding the other road user who is in the priority position due to your lack of observations.
At one of the extreme to other driver may have had to reduce speed very slightly to accommodate you. This on its own would not be a warrant a cross on the Statement of Failure. More conversely if your action causes the other driver to brake to significantly reduce the speed this would be deemed potentially dangerous and result in a cross on the statement of failure. At the other most extreme if the other driver has to brake heavily and swerve or indeed there is any collision this would be assessed as dangerous result in a cross on the Statement of the Failure and be written up as a dangerous (D) occurrence in the examiner's report.
The other fault mentioned was about Speed on approach to junctions. Suppose if this is marked as a cross it is most likely that you have approached the junction far too fast even though you have every intention of stopping at the junction and indeed do so. This could also cause the drivers on the major road considerable concern because of your Speed on approach to the junction you may just have caused them to reduce speed due to your driving technique which has left a lot to be desired.
Another instance for the marking would be if you approach a junction within less time not giving yourself time to take proper observation before emerging thus causing concern to other drivers in the priority position. In this kind of instance you would most likely have had both the faults marked with a cross. Any one of them fault considered dangerous or potentially dangerous is enough to fail your test. This does happen and rightly soIf at the end of the test you are not satisfied with the assessments you can write in and appeal. Anyhow because it is difficult to recreate the situations that arose on the test- it is very unlikely the decision of the examiners will be changed.