In terms of web site marketing there's one fast way to lose credibility on your site.

Unless you've got a podcast or video on your site the only interaction your visitor has are the words on the page.

Therefore, with nothing else to convince them that your site is trustworthy, the fastest way to lose a visitor is as follows:

Let's say you've got a headline that reads something like, "We sell loads of stuff to loads of customets"

You will notice that customers is spelt incorrectly.

All of the major studies done on web site design and usability tell us that a minor grammatical error can deter a sale, and your visitor will leave.

Make sure all of your marketing collateral, blogs, and newsletters are spell checked for typos otherwise you're simple wasting your time.