Perhaps the most common error I see in proofreading is the choice of the wrong dash. Frequently, a hyphen or suspended hyphen—once known as the "secretary's friend"—is substituted. Reference manuals devote page after page to the various types and uses, but here's a brief summary:
En Dash –
To mean "to" or indicate a range between figures or words. It is half the length of an em dash, but longer than a hyphen. Examples:
2009–2010; pages 5–15; open 10 a.m.–6 p.m.; chapters 2–3
An en dash can also be used to set off bullet points.
Em Dash —
Properly used to mark a suspension of the sense, a faltering in speech, an appositive, a sudden change in construction or an unexpected turn of thought.* See the first paragraph of this post for one example; here are others:
What are you doing here with the flu—go home!
Warning: Don't overuse the dash—if a comma would work, use the comma.
For more information and tips on creating the correct size on your own keyboard:
http://desktoppub.about.com/cs/finetypography/ht/dashes_hyphens.htm
http://en.wikipedia.org/wiki/Dash#En_dash_versus_em_dash
Don't forget to include each client's dash preferences in their style sheet!