It's possible to do your revision so well that you think you know your subject inside out, back to front, and every which way. That's brilliant! But just remember, in the heat of the exam, you still need to be certain you answer every question properly. If you memorize essays ready to write them back in an exam, be sure that the essay you write is the one the examiner wants to read—not the essay you just happen to have revised. So take time to read and understand the questions on the exam paper. The more senior you are, the more likely you are to find that essay questions on exam papers aren't completely straightforward. You may need to read between the lines before you realize "Oh yes, this is actually a question about x/y/z. They've mentioned such-and-such, but what they're really asking me about is so-and-so". Again, practicing past exam papers will help you to recognize what you're being asked and demonstrate your knowledge in the way the examiner is expecting.