Your candidate number changes every academic year. We know that sometimes students recall a number from a previous year and use this in error. If you use the wrong candidate number, then please contact your department/faculty as soon as possible to make them aware of your error.
Remember to check your candidate number carefully before entering the exam hall, and especially for your first exam period of a new academic year. If you need to know how to find it, please see our article What is my candidate number & where can I find it?