- Two spaces between sentences. Your keyboarding teacher not only taught you how to type, but also how to sin. Once upon a time, typewriters used a monospaced typeface. Since all of the letters were the same width, it became customary to add an extra space at the end of a sentence to call attention to a new sentence. This was never the practice of professional typesetters, who always used one space. (If you're a doubting Thomas, go find an old book and see for yourself.) Since most typefaces on our computers vary in width, unsightly gaps appear if two spaces are used. Repent of this since by using only one space.
- Failing to kern display type. Nothing bellows "I'm an amateur!" quite like display type that hasn't been properly kerned. Unseemly gaps can impede readability by distracting the reader. The kerning tables of some typefaces are great, but the human eye is divine. Adjust the spacing between letters and assuage your guilt.
- Using a hyphen instead of an en dash. A dash (hyphen) is great for a hyphenated word, but an en dash can be used to indicate a range of numbers or a duration of time instead of the word "to": the 8–10 Commandments, not 8-10 Commandments.
—From "Thou Shalt Not" by Jim Godfrey, published in the July 2011 issue of How magazine.