im guessing this isnt as level maths as i havent done mine yet, but in response to your question barcodes would need a large hole to make it un-readable. and lets say you brought water into the exam and miraculously only spilt water on the barcode, as long as your centre/candidate number can be read it'll be fine as they can check what exams you've taken to ensure that the paper is yours. so i shouldnt worry if i were you