1. Misused apostrophes. Using “it’s” (the contraction for “it is”) instead of “its” (the possessive) is basic, yet it somehow happens all the time. Keep an eye on apostrophes to avoid an inexcusable grammar mistake, as in: they’re and their (or there), who’s and whose or you’re and your.
2. To “Comprise.” Since “comprise” means “to consist of,” it’s never OK to say “comprised of.” The word is so commonly misused—and by prominent people in communications who should know better—that I fear the incorrect usage will slowly make its way into the lexicon.
3. Overused keywords. In the digital age, keyword stuffing can be a fatal flaw for PR writing that lives on the Web. Since most original content, such as blog posts, product descriptions and narratives, tweets, and captions, is published online, this is a mistake to be avoided, lest search engines ignore your content completely.