You probably have no idea that it's happening.
But when you open your mouth, or let your fingers dance across the keyboard, you may be killing your own credibility in the ears and eyes of whomever you are trying to impress.
In many cases, you know exactly what you want to communicate to someone, but for whatever reason, you have an inability to communicate it properly. And that can send doubts racing through their mind.
The result may be that you'll achieve the very opposite of what you intended to do.
You won't get that contract.
You won't get that promotion.
You won't get hired.