The 'instructors' basically reserve the right to fail you at any given point throughout the test. There are dozens of little things that they can cite as point deductions, but if they 'like' you, they just leave the deduction blank and go to the next part of the test.
There is a class (at least there was when I took it in 1992) offered at CCRI that prepares you for both the written and driving portions, I highly recommend it.
During the driving portion, DO NOT hit the cone on the right hand turn (I've heard that it's an automatic fail, and it happened to my brother), and also keep in mind that you can take pull ups in the parallel parking for little or no deduction, definitely less than hitting the cone. Also, be as thorough as possible with the walk around inspection and air brake portion of the test.
There's also some weird thing about not shutting the truck off until they say so. And don't forget to chock the wheels!
-Tom