The usage issues that follow are not so much pet peeves as they are mistakes that I encounter constantly and have seen most commonly in the past year or so.

"Over-pluralizing"
In recent years there seems to be a growing misunderstanding about agreement in number between a plural noun and other related nouns that follow in the sentence. English is not like Spanish, for example, in which agreement in number is broadly required: You don't say "mucha gracias" because gracias is plural and therefore the adjective describing it must be plural: muchas gracias.

Here's what Words Into Type* says under the heading Singular with a Plural Possessive:

*Words Into Type, Third Edition, Prentice Hall

Study the following examples, all of which were gleaned from newspapers, magazines or heard on radio/TV:

• Poor: Both players are making their debuts tonight with the Heat.
• Better: Both players are making their debut tonight with the Heat.

• Poor: He argued that survivors at the camps with no place to go should be sent — even against their wills — to what would later become the state of Israel.
• Better: He argued that survivors at the camps with no place to go should be sent — even against their will — to what would later become the state of Israel.

• Poor: Many college students are taking classes now, hoping to hasten their graduations.
• Better: Many college students are taking classes now, hoping to hasten their graduation.

• Poor: Two Nissan vehicles ranked at the tops of their classes in the latest J.D. Power and Associates study.
• Better: Two Nissan vehicles ranked at the top of their class in the latest J.D. Power and Associates study.

Based on the principle "only one of the things possessed could belong to each individual," the same rule applies to the following examples that do not include a plural possessive:

• Poor: Those placid scenes were a sharp contrast from the high-crime reputations the parks developed in recent decades.
• Better: Those placid scenes were a sharp contrast from the high-crime reputation the parks developed in recent decades. (each park can only have one reputation)

• Poor: Payload is the GVWR minus curb weight and includes weights of occupants, optional equipment and cargo.
• Better: Payload is the GVWR minus curb weight and includes weight of occupants, optional equipment and cargo. (combined weight of occupants, optional equipment and cargo is implied; weight does not need nor want an s)

This mistake is grating on the ears and perpetuates a misunderstanding of the language; the careful editor should not let it pass.

who vs. that
Use who for people and animals with a name. Use that for inanimate objects and animals without a name. Errors like the ones below are absolutely rampant these days!

• Wrong: "Turns out he was an oncologist that made $500,000 a year."
• Right: "Turns out he was an oncologist who made $500,000 a year."

• Wrong: This is a character that has to become emotionally involved in a case to successfully defend it.
• Right: This is a character who has to become emotionally involved in a case to successfully defend it.

collective nouns
Collective nouns are words that indicate a group, such as class, committee, crowd, family, group, herd, jury, band, orchestra, couple, population, staff and team. A collective noun takes a singular verb unless the individuals forming the group are to be emphasized.

• The group has decided.
• The family has just moved in.
• The population of the U.S. is diverse.
• The committee adheres to its decision.

But:

• The committee have signed their names to the report.
• The couple were married in 1989.

The error I see most commonly in this category is using a singular verb with couple, pair or duo when a plural verb would be more appropriate. Here's a clue: if the sentence later refers to the couple/pair/duo as they, then the plural verb should definitely be used.

• Weak: Once the pair gets a look at Granddad's prized car, they steal it for their getaway.
• Better: Once the pair get a look at Granddad's prized car, they steal it for their getaway.

apostrophes turning the wrong way
Word processing programs often will reproduce a single open quote (‘) when you type an apostrophe (’). Notice that in most fonts they turn different ways — a single open quote turns up and to the right, an apostrophe turns down and to the left. Or if they don't curve, at least the apostrophe is wider at the top than at the bottom, and the single open quote is wider at the bottom than at the top. Frequently when a file has been imported from another program or platform, apostrophes unfortunately tend to get transformed into single quotes, and curving (“smart”) quotes and apostrophes often get “dumbed down” into straight quotes/apostrophes. Watch for this: it should be ’09, not ‘09 or ′09.

• Wrong: NATPE ‘08
• Right: NATPE ’08

• Wrong: That ‘70s Show
• Right: That ’70s Show

• Wrong: ‘Til Death
• Right: ’Til Death

ensure/insure/assure
Ensure means to guarantee. Insure is only used in reference to insurance and the insurance industry. Assure means to promise.

• The policy insures his life.
• Steps were taken to ensure accuracy.
• I assure you I will be there.

Note the use of "that" for readability and to avoid confusion.

• Poor: Save the e-mail address to ensure you return the file to its source.
• Better: Save the e-mail address to ensure that you return the file to its source.

that/which
The popular misconception is that which is more literary and therefore preferable. That is used to introduce an essential or defining clause (also called restrictive) — a clause essential to the meaning of the sentence.

• Wrong: The car which I chose was red.
• Right: The car that I chose was red.
• Or: The car I chose was red.

Which is used to introduce a nonessential or nondefining clause. The clause introduced by which contains nonessential information — the sentence can stand alone without it. Clauses introduced by which can be set off with commas. Good rule of thumb: If the clause can't be set off by commas, use that, not which.

• The book I gave Sandra, which she liked, was written by Mark Twain.
• The other bill, which is sure to be controversial, will be voted on next session.

Use the correct preposition!

• Poor: It's been a long journey since four young engineers arrived to California.
• Better: It's been a long journey since four young engineers arrived in California.

• Poor: in this epic third installment to the blockbuster feature film franchise.
• Better: in this epic third installment of the blockbuster feature film franchise.

• Poor: Bogart was among the many actors and directors who formed their own production companies in the postwar years.
• Better: Bogart was among the many actors and directors who formed their own production companies during the postwar years.

• Poor: Three businessmen with last names starting in D, A and T
• Better: Three businessmen with last names starting with D, A and T

• Poor: . . . and collaborated on the music of A Mighty Wind.
• Better: . . . and collaborated on the music for A Mighty Wind.

affect/effect
Affect is a verb meaning to influence. Effect is a noun meaning cause or result.

• The introduction of Apple's iPhone will affect everything else in the media marketplace.
• He miscalculated the effect of his actions.

its/it's
Its is the possessive form of it, meaning "belonging to it." It's is a contraction for it is.

• It's up to you.
• The company lost its assets.

spacing
Use only one space between sentences (after periods and colons). Double-spacing after periods and colons was the rule in the old typewriter-and-Courier-typeface days, but for printed matter a single space is standard.