Friend got some crazy mark in A Level History and she revised on the train on the way there, she wasn't lazy just already had the A in the bag, turned out she could have done it for all exams.
Not strictly speaking cramming but a mate was literally assured an A in Biology (back in the days of 6 units he'd done 5 and got full marks in the lot, putting him on exactly 80% with the summative to go) and turned up to the only summer Bio exam, his last, basically still absolutely hammered from the night out to celebrate the last exam he had which counted the night before, think he'd had a can to 'take the edge off the hangover'. He got 89% in that exam, he's going to be a doctor in a year and a bit, God help us all.