As a Wikipedian, by far the most common edits I have made are fixes to incorrect punctuation. Here are the mistakes I see most often. This list is not comprehensive because it is restricted to the errors I encounter most frequently. I have also tried to restrict the list to things that are universally regarded as incorrect, as opposed to stylistic issues like the Oxford comma.
Some examples were taken from Wikipedia articles, whether they're actual errors I found or they were used in an article as a sentence illustrating an error. Others were made up on the spot.
Commas
Here is a fundamental rule of comma usage that is often neglected:
When you have an opening comma, you must also have a closing comma. The only exception is when the closing comma would come after the last word in the sentence.
- Wrong: Joe, who goes to the gym every week is quite strong.
- Right: Joe, who goes to the gym every week, is quite strong.
Commas are not necessary:
- With titles of books, movies, TV shows, TV episodes, newspapers, etc.
- Wrong: The TV series, Star Trek, began in 1966.
- Right: The TV series Star Trek began in 1966.
- But contrast: William Shatner became the star of a new series. That series, Star Trek, began in 1966. In the previous example, "Star Trek" was the subject of the sentence and the phrase "TV series" was merely clarifying what Star Trek is. In this second example, however, "series" is the subject of the sentence, and "Star Trek" is just additional information clarifying what the series in question is.
- Before parentheses. See the section on parentheses.
- After the subject of the sentence. I call this the "dramatic pause comma". Just because you might insert a pause in speech does not mean a comma belongs.
- Wrong: Peacocks, are birds with funny tails.
- Right: Peacocks are birds with funny tails.
Avoid comma splices: do not use commas where it is more appropriate to use a semicolon, colon, or dash. Sometimes it's best to add a conjunction after the comma or split the sentence into two sentences.
- Wrong: It is nearly half past five, we cannot reach town before dark.
- Right: We cannot reach town before dark: it is nearly half past five.
- Right: It is nearly half past five; we cannot reach town before dark.
- Right: It is nearly half past five, so we cannot reach town before dark.
- Right: It is nearly half past five. We cannot reach town before dark.
(The article on comma splices has a few more variations of this sentence, but this should be enough to illustrate the idea.)
Parentheses
If you wouldn't use the punctuation without the parenthesized phrase, then there shouldn't be any around the parentheses.
- Wrong: She talks all the time, (which annoys me). If you remove the parenthesized phrase, you get "She talks all the time,."
- Right: She talks all the time (which annoys me).
- Right: She talks all the time, which annoys me.
A sentence that begins outside parentheses ends outside parentheses.
- Wrong: She talks all the time (which annoys me.) If you remove the parenthesized phrase, you get "She talks all the time" — not a complete sentence without its period.
- Right: She talks all the time (which annoys me).
A sentence that begins inside parentheses ends inside parentheses.
- Wrong: (This annoys me).
- Right: (This annoys me.)
What do you do if you want to have an entire sentence inside parentheses that itself exists in another sentence? In such a case, you probably should rephrase to avoid the problem.
Quotation marks
Always nest quotation marks correctly: single quotes within double quotes and vice versa.
- Wrong: Derrida said that the aim of the US media was "to discredit what is considered the exorbitant and cumbersome "credit" of a foreign professor."
- Wrong: Derrida said that the aim of the US media was “to discredit what is considered the exorbitant and cumbersome "credit" of a foreign professor.” Interchanging curly and straight quotation marks does not count. You must use double within single or single within double.
- Right: Derrida said that the aim of the US media was "to discredit what is considered the exorbitant and cumbersome 'credit' of a foreign professor."
Do not put a commas after a quotation that ends with a period. Replace the period with a comma.
- Wrong: "I'll do that.", said Jane.
- Right: "I'll do that," said Jane.
Note how the comma goes inside the quotation marks. When the comma stands in for a period, this rule applies even if you're using the so-called "logical" or "British" style.
Finally, there is no need to put quotation marks around a passage that is blockquoted.
Semicolons
Semicolons are often used where commas or colons would be more appropriate (but be sure to avoid comma splices).
- Wrong: They are composed of yellow, magenta, and cyan organic dyes; which fade at different rates.
- Right: They are composed of yellow, magenta, and cyan organic dyes, which fade at different rates.
This is a sentence pulled from an actual Wikipedia article (albeit trimmed for brevity). Its author might have been thinking the semicolon is necessary since commas were used to delimit the items of the list, but it's not, since the word "and" clearly signals the last item of the list.