Not really a high mark (well, it is for me) but in GCSE Maths I gained a C! What's the big deal you ask? Well I'd never ever achieved a C in Maths. Recent mocks were poor (U's and E's) and I'd never attained a C in any progress report etc. Of course, I thought I was doomed to failure. However, a miracle seemed to take place and I passed! The first time I ever got a C was in the actual exam, what are the chances eh?!
The rest of my grades I'd been roughly predicted close enough to and came out overall with A*A*A*ABCCCC, so I'd say that mostly your predicted grades will be quite near the real grade you'll get.