Indenting the first paragraph.
The first paragraph is never indented, subsequent paragraphs are.
Indenting a paragraph too far.
The standard indent for a paragraph is 1 em, not ½ inch. (Identing and adding empty space require penance. Save your soul by using one or the other, not both.)
Failing to hang punctuation into the margin.
Punctuation has less visual weight than letters or numbers. Compensate for this in display text by hanging the punctuation into the margin.
Failing to use or create fractions.
Wicked: ½ § Righteous: ½

See them all at: Typographic Sins → Teaching → Ray Elder via: SwissMiss