Wrong candidate number!!Watch this thread
Before sending off all the papers to the exam board there's a process they go through of checking the details on the front page. If they spot an error they'll simply correct it. (The front page is the only page they're allowed to correct, for obvious reasons!)
However teachers are humans too, and if the mistake is subtle (a single digit being wrong, for example) they might not spot it. So definitely contact the school to make them aware, so they can specifically look-out for your paper.
Our Cambridge officer told us the whole process, and apparently; the package containing all the papers for the exams that were conducted that day must leave the school that day itself. And I’m pretty sure they count the papers and seal the packages before you leave the exam hall, so that they cannot be tampered with. But each centre handles the packaging part of the process differently so I don’t know.
So inform someone asap so that they can handle it right now, or atleast tell you what will happen so you don’t worry so much