Remember that editing your written work is an essential part of the writing process. At this level, you should be able to spot both basic errors and more complex ones. This is a part of the process of writing. In the CPE/CAE/FCE exams, finding and correcting a single error might be the difference between a pass and a fail in the written paper. It could also be the difference between an A and a B.
You should leave yourself at least 5 minutes to edit each piece of writing in the exam. 10 minutes would be better!