You’ve finished your short story or novel and it’s wonderful. It is the most exciting piece of writing to ever grace a computer screen and the best bit is, there are no mistakes. Okay, maybe one or too, but the story is so good that the reader won’t notice or will forgive you. But if you’re sending it out to an agent, publisher, or competition, or submitting it to a writing group, that assumption is so wrong, it couldn’t be wronger if it tried.
(So far so good, but please tell me that you noticed the word ‘too’ in the paragraph above. If you didn’t, you really need to practice your proofreading/editing skills. You could try ‘Writers’ Forum Magazine’ which has a monthly feature titled ‘First Draft’ that shows a section from a published book with intentional mistakes for you to find.)