The examiner will not try to trick you, certainly not in the UK under JAR. I can't speak for anywhere else, but i doubt that they would try to deliberately fool or force you into making an error.
My CPL examiner told me that "Everyone walks into that briefing room with a pass; whether or not they still have that pass by the end of the day is up to them".
As far as practical advice, make sure your organised. I had a ring binder for my CPL and IR test with specific divisions for Weather, Notams, Spare PLOGs, M&B info, and a folder for all test applications, confirmations, receipts, etc. Basically all the bumff that you needed to present to the examiner for the test was in this folder. That seemed to go down well. Ask questions if you are not sure of anything. Better to ask and know than to keep quiet and make a silly mistake...
Make sure you eat something before the test. If you are anything like me your stomach will be doing somersaults before you go in, but you will need some blood sugar to aid concentration, etc. Adrenaline alone won't keep you going!
Dont panic if you make a mistake, or see the examiner writing stuff. Chances are it is some minor debrief point, or (as happened on my CPL) a reminder for the examiner to buy a loaf of bread and some milk on the way home!
If the worst happens and you do make a serious balls up, keep calm. All is not lost. Put it to one side and get on with the rest of the test, maybe you can't pass it, but you can still fail it. Go for the Partial.
TTR