Why this is a mistake: Most writers have a few blind spots when it comes to their work, and these negatives tend to come out in their business dealings. Insecurities are often most obvious in submissions and proposals, as in using the copyright symbol on cover letters and cover sheets for manuscripts. Writers might be quick to point out that they need to protect their work. But what they imply by using the copyright symbol is that they fear the agent or editor they are sending their work to will steal their idea.
In the same way, many authors put subconscious negative comments in their cover letters. They will unwittingly say negative things about themselves or their writing, mentioning that this is their first article or book, for instance. Or they will slam the genre or form in which they are writing. Some of them even say negative things about the agent or editor to whom they are submitting.
The solution: Review any written material before you send it, and examine it sentence by sentence for subconscious negatives. Give it to someone else to read and have the person look for these negatives. Examine your word choice. Examine your verbs. Do not put anything in writing that can be taken as a negative, either consciously or subconsciously.
Ask yourself how you would feel reading your material cold without any idea who the author of it was. What would be your initial impression?
(Remember that the initial impression is often the only impression.) Would it be positive or negative? Make sure it is a positive one.