Copyediting ensures that your document is considered both professional and coherent by eliminating the following:

Misspellings (including the ones overlooked by spell-checking programs)
Grammatical errors
Stylistic problems, such as capitalization and punctuation issues
Typos
Problems of usage
Problems of parallelism
Discontinuities
Ambiguities
Triteness
Awkward phrasing
Problems with references

The copyeditor’s job is to “decide which kinks or knots in someone else’s writing seem likely to disrupt communication with the intended readers and then to revise those patches as unobtrusively as possible” (Amy Einsohn, The Copyeditor’s Handbook: A Guide for Book Publishing and Corporate Communications).

Did you catch that phrase “as unobtrusively as possible”? I regard a writer’s task as putting profound thoughts onto paper, a copyeditor’s task as merely polishing, ensuring that those profound thoughts reach a reader’s mind with as little noise (clutter from inconsistencies or problems with grammar and syntax) as possible—always without revising (messing with) the writer’s authorial voice. Every writer, including the best ones, need an editor (and I need an editor as well whenever I write). But, all my markup aside, I harbor no pretension that I could have written the writer’s work myself. Still, even excellent writing, if unedited, will disappoint readers, who will unfairly disparage it because of its lack of nitpicking polishing.

Here’s what Holly Robinson, author of novels Sleeping Tigers and The Wishing Hill (as well as magazine pieces on pop culture, parenting, health, science, and psychology), has to say about copyediting:

“I crossed out ‘Tuesday’ because later you say it’s Wednesday.”

“She's fifty-nine here and fifty-eight on page 102. Which one?”

“If he Googles the land line, why is she answering the call on her cell phone?”

I’m going through the copy editor’s remarks on my new manuscript—the one that will be published by Penguin Random House as Haven Lake in April 2015. And, once again, I can’t believe all the mistakes I made in this book—even after eight or nine revisions, two of which were done in collaboration with my savvy, brilliant editor.…

[A] copy editor is someone who takes out [his] bright lamp, microscope, and fine-toothed comb. [He] nit-picks through each one of your pages, catching time transitions that don’t make sense, erroneous spellings, accent marks if one of your characters happens to speak a foreign language, word repetitions, name changes or hair color changes you forgot you made, etc. In other words, the copy editor is a fierce, mistake-seeking hound, nosing around in every dark corner of every paragraph to make sure you get things right.…

[For] those of you who are self-publishing books, some advice: if you have any extra funds, do yourselves a favor and hire a copy editor. Your books—and your reputation as a writer—will be better because of it.

Now back to my manuscript and the copy editor’s bubble comments in the margin:

“It can’t be Saturday here, because you said it was a school day earlier.”

“Same words in previous sentence. Change here?”

As John Cleese of As Michael Palin of Monty Python would say, “My brain hurts.” [Here is an actual example. The correction comes from copyeditor Mark Harvey, who was one of the several copyeditors posting grateful comments in response to Robinson’s laud. He points out that after Palin’s complaint, Cleese replied: “My brain hurts, too.”] But it’s so worth it. No manuscript will ever be perfect. But, thanks to copy editors, we can get closer.

—Holly Robinson, “In Praise of Copy Editors, Publishing’s Unsung Heroes”
(For another laud of copyeditors, see this one, from author Roy Peter Clark)

[Did you notice that Robinson uses the two-word copy editor in her laud, not the currently preferred single-word copyeditor (sanctioned by the publishing industry’s “bible,” the University of Chicago Manual of Style)? Allowing this discrepancy is actually another example of copyediting; we copyeditors need to leave quoted material as is. (I could have inserted the somewhat denunciatory “[sic]” with Robinson’s spelling, but I deem such a challenge distracting from her kind remarks.)]

You can see a series of copyediting samples, or you can examine any specific one from the following list of problems that copyediting resolved.

Online samples

1. He died in obscurity (problem with incorrect statement of fact) Fiction
2. Gangster escapades (problems with continuity, clarity, and emphasis) Fiction
3. Too many Steves (problems with continuity) Fiction
4. Tri or Quad (problems with consistency and statements of fact) Fiction
5. Apologizing to himself? (problem with continuity—a character addressing himself?— and static dialogue) Fiction
6. The escape (problems with word and even punctuation choice) Fiction
7. A barbecue pit (problems with spelling and grammar) Fiction
8. How old is Stella? (problem with continuity: the the years don’t add up) Fiction
9. The road was clogged (problems with coherence and emphasis, improved with passive voice) Fiction
10. Truck or SUV? (or car?) (problems with continuity and tense) Fiction
11. People lined up (problems with weak, inactive language) Fiction
12. Upstream (problem with logic and continuity) Fiction
13. Who drove whom? (problem with continuity) Fiction
14. How long ago? (problem with continuity) Fiction
15. Hi from her (problem with intended meaning) Fiction
16. President or professor (problem with ambiguity) Fiction
17. Frank stuffing French fries fresh from the fryer (problems with risible juxtapositions) Fiction
18. At the run-down motel (problems with continuity and dialogue logic) Fiction
19. Useless politicians (problems with ambiguity, word choice, and muddied dialogue) Fiction
20. At the Capricorn Lounge (problems with accuracy and precision) Fiction
21. Intensity in the driveway (problems with rhetorical parallelism) Fiction
22. Up to date with updates (problems with awkward phrasing and mechanical style) Fiction
23. Brice Prairie (problems with logic and vagueness) Fiction
24. First contact (problems with mechanical style, syntax, and word choice) Fiction
25. Kicking the dog’s bowl (problems with ambiguity and context) Fiction
26. Avoiding full-tilt panic (problems with mechanical style) Fiction
27. Semiotic film theory (problems with mechanical style and parallelism) Fiction
28. Four stories (problems with continuity) Fiction
29. Boots (problems with continuity in word choice) Fiction
30. Forty-plus years (problems with syntax and word choice) Fiction
31. The toddler daughter (problems with continuity and paragraphing) Fiction
32. The mage tower (problems with discrepancies, coherence, vagueness, and mechanical style) Fiction
33. The victory arch (an ambiguous construction and imprecise word choice) Fiction
34. A bloodthirsty look in his eyes (problem with awkward word choice) Fiction
35. With hungry eyes (problem with flow) Fiction
36. A vampire’s birthday (problems with punctuation, tense, word order, and sentence structure) Fiction
37. An airborne game warden (problems with mechanical style) Fiction
38. The wrong bridge (problems with easily verified facts) Fiction
39. Handhold and handset (problems with word choice, imprecise phrases, and static dialogue) Fiction
40. Seven children total (problems with simple arithmetic) Memoir
41. Ford, DeSoto, and then Chevrolet (problems with redundancy, coherence, and a point that needed addressing) Memoir
42. The Scoop nickname (problem with an incorrect statement of fact; also awkward phrases and mechanical style matters) Memoir
43. Until his retirement? (problem with an internal contradiction) Memoir
44. Events in order (text out of coherent chronological order) Memoir
45. Arranging names (spelling of proper names, alphabetizing of lists) Memoir
46. A signifiant subchapter (discrepancy from one chapter to another, correction of facts) Memoir
47. Friederike (problems with mechanical style and facts) Memoir
48. Learning Italian (problems with mechanical style and facts) Memoir
49. The “chronic liar” (problems with mechanical style, place-names, and ambiguity) Memoir
50. East Germany (problems with mechanical style and lack of clarity) Memoir
51. Napoleon returns to Buenos Aires? (problems with mechanical style and syntax) Memoir
52. Correcting place-names (problems with mechanical style and place-names) Memoir
53. A 1990 graduate (problems with syntax, awkward phrasing, and facts) Memoir
54. Qur’anic exegesis (problems with scriptural citation, transliteration, word choice, and usage) Religious (Muslim)
55. Person markers (problems with inconsistency, word choice, verbose phrasing, and style) Academic (linguistics)
56. Development of the Human Genome Project (a problem with a reference) Medical
57. Behavioral phenotype (nonconformance to APA style) Medical
58. Positive support and intervention (more nonconformance to APA style) Medical
59. Medical risks (an apparent contradiction) Medical
60. References (a sloppy reference list) Medical
61. Musical ability and Williams syndrome (little details) Medical
62. Resources (a sloppy list of names and addresses) Medical
63. What does it take to succeed? (problems with modifiers and referents) Popular how-to
64. Exposition (an apparent contradiction and other problems) Screen play

Hard-copy samples

65. Installing the infrared device driver (an unresolved alias) Technical
66. The HelpClub numbers in Europe (an untranslated list) Technical
67. Setting the conditions for extraction (a wrong choice of words) Technical
68. How long a battery will last (some extraneous information) Technical
69. Keeping away from children (an ambiguous danger notice) Technical
70. Dragging a mouse (a poor choice of words) Technical
71. What is a browser? (another poor choice of words) Technical
72. Starting the CE Tool test (a dangling modifier and a usage problem) Technical
73. Installing a modem (a misleading conjunction) Technical
74. Attaching a mouse (another misleading conjunction) Technical
75. The ThinkPad online book (some awkward phrasing) Technical
76. Caring for the LCD (some redundancies) Technical
77. Assistance in your daily life (more awkward phrasing) Technical
78. Conventions for messages in the text (a discontinuity) Technical
79. InfoAnalyzer operation (a misplaced modifier and some word choice issues) Technical