Route planning

Poor route planning will lead to the learner making mistakes that are, in reality, the fault of the supervisor. 

When routes are poorly planned it's virtually impossible to assess the driving accurately. There is also a good chance that you will end up in an argument with the person you are supervising or with another driver!

It will be difficult to distinguish between those mistakes that are made because of the route 'pressure' and those which occur because of a lack of skill or understanding. 

Inappropriate routes will lead to lack of motivation and a slow learning rate (and lots of arguments!).

Another problem with unsuitable routes is that many of the other drivers you encounter will be unforgiving of a car displaying 'L' plates.

While there is an ever present risk of 'road-rage' in these situations, perhaps the biggest problem would be the negative feelings of the learner (embarrassment after stalling, for example) which will zap confidence.