I usually make the following common mistakes when I need to use APA sixth edition and US English. I got these comments from an editing service.
In APA style, “et al.” is not italicized.
If a quotation is set off by quotation marks, there is no need to italicize it.
In U.S. English usage, periods (full stops) and commas always go inside the closing quotation mark.
“Firstly,” “secondly,” and “thirdly” are common UK English but virtually unheard of in US English.
APA style calls for Level 4 headings to be set in boldface italic.
While “towards” is common in UK English, “toward” is generally preferred in US English.