I can remember a great example where a women entrusted her son to proofread her resume for errors and accuracy. After one month of not getting any leads for work she read over her resume. It was saturated with typos and slang. I have also come across bands who write "We are the next best thing" or "If you don't hire us you are a fool." This might have sounded like a good idea to these bands at the time. In truth, the terms they chose sounded desperate, slightly arrogant, and condescending to whomever they were approaching for potential work. I have had other business clients write pitch letters making claims such as "You'll never use another product like this again" and "Guaranteed to work." These are pitfall phrases. Should one ever find oneself in a position of crisis communication (such as a faulty product) these statements will act as linchpins.
Promotional writing is a fine blend of boastful truth and concise communication. These elements are the basis for writing any promotional copy. Grammar is important but the tone of delivery is too!