I work at a top nonfiction literary agency and have the privilege of working with some supremely talented writers, including PEN/Faulkner and Pulitzer Prize winners. Part of my job is to give their proposals a final once-over copyedit before sending them out to publishing houses.  

I have been here for three years, and have never seen an error-free manuscript.

For the last year or so, I've been keeping a running list of the most frequent errors I see these talented writers make. It's proof that even the most successful authors miss things in their own writing.
I always say that trying to copyedit your own writing is like trying to lick your own elbow--you can get close, but in the end, it's a lot more efficient to have someone else do it.  


Keep an eye out for these bugaboos in your own writing, and if you spy them, know you're in good company.

1.   Confusing Restrictive and Nonrestrictive Clauses.
Example Problem Sentence: "I took my wife Sandy to the park."
Explanation of problem: Does the author have more than one wife?  If so, then this sentence is fine. But if he's not a bigamist, then he owes Sandy an apology--and some commas around her name to prove she's his one and only. (Still confused? Check out Grammar Girl's in-depth explanation.)

2.   Hyphens with -ly Adverbs
Example: "The rapidly-expanding company had finally hit a wall."
Explanation: You don't need a hyphen between an -ly adverb and an adjective. It's unnecessary because the adverb ("rapidly") can only be modifying the adjective ("expanding"), so there's no need to use a hyphen to show what the adverb is modifying.
Use a hyphen with adverbs that can sometimes be used as adjectives, like "well," to avoid confusion: the well-dressed man is a man who is dressed well, rather than a man who is neither ill nor nude.

3.   Rogue Hyphens with Ages
Example:  "When she was sixteen-years old, Rita graduated as the valedictorian of her high school class."
Explanation: Even authors who usually understand how to use hyphens get tripped up by ages. Just like other adjectives, ages need a hyphen only when they come before the noun they're modifying, e.g., "Rita was a sixteen-year-old girl."  (The sort-of exception: you still need hyphens when the noun is implied but not explicitly written, as in "Rita was a sixteen-year-old.")  

Similarly, even authors who use hyphens correctly most of the time often trip over "[X]-year-old [noun]." 
For example: "He shipped out of Camp Lejeune with the other 18 year-old Marines."
 "Marines" is the noun being modified. Here, it's being modified by "year-old" --not "18-year-old". Unless the Marines are now recruiting toddlers, that was not the author's intended meaning.

4.   At Wits End
Example: "Isolated and scared, Pat was at his wits end."
If you are at the end of your wits, you are at your wits' end (or wit's end--both are acceptable). 

5.   Hyphens with compound adjectives
Example: "In 1980, the Supreme Court would overturn this by allowing a crude oil-eating bacterium to be patented."
If this author meant to write about an oil-eating bacteria with a penchant for lewd jokes, this is just fine. But if she meant to signify that the bacterium eats crude oil, we're in trouble.
That's because in the example sentence, there's just one hyphen, and it's connecting "oil" with "eating," not connecting the compound noun "crude oil" with "eating". It needs another hyphen to link everything up: "a crude-oil-eating bacterium".
For adjectives that include an unhyphenated/open compound noun, like "Pulitzer Prize–winning play", call in the big guns: the en dash.