1st time: messed up my parallel parking so badly the examiner had to direct me out. That was 10 minutes in, and he made me do the entire rest of the test despite me asking to go back to the test centre. I said "I know I've failed now, can we go back please?", he said "You never know. And no, carry on please.". Douchebag.
2nd time: Wasn't far enough to the right at the junction leading into the test centre on the way back. I had one minor apart from that, grr.
3rd time: I was a bit wide on my reverse round the corner. It was unusually sharp and the examiner even said if I'd been two inches nearer the kerb I would have passed.
4th time I passed. Practise makes perfect eh?