This is a catch-all place to note and record common errors in usage, together with the correct form. Such usage errors are legion. As time goes by, the listing will probably become much longer. To start off, here are a few, which I jotted down after listening to the casual speech of other persons.

“skim” and “scan” — These two words are opposites: To “skim” a passage is to read it quickly and superficially. To “scan” a passage means to read it carefully. For some perverse reason, otherwise informed people often think these words are synonyms. Get this straight: “scan” means to be careful, not superficial. If you have trouble with this, remember that a “scanner” captures every detail of the item scanned.
Perhaps it is also helpful to recall that “to skim” means to touch just the surface of something, as in skimming a pebble across the water. Sadly, even authors and editors occasionally commit this error, especially in using “scan” to mean a quick once-over.
“could potentially” — The “potentially” is redundant. Strictly speaking, “could” means “was able to” (as in “He could play the Moonlight Sonata when he was 13.“). As such, it is the past tense of “can.” In these times, however, it is also used as a synonym for “might,” when referring to something unknown or doubtful: “It could snow this afternoon.” In general, “might” is preferable because it is the correct word, and “could” is not; but if you use “could” to express uncertainty, don’t compound the sin by adding the gratuitous “potentially.”
“each” and “both” — Sometimes, when you’re mindlessly chatting in your native tongue without thinking very hard about it, you will come up against the “each vs. both” problem. Remember to stop and think it through. If you do, you are unlikely to get this wrong. For example, if you say, “They both have wives,” it means, literally, that they are polygamous, and possibly with the same women! If you mean that each one is married, and not to the same person, you say, “Each one has a wife.” Whether you use the singular or plural depends on the subject of the sentence (not on the word “each” or “both”). If the subject is “they,” or if it is a conjoint compound subject (more than one person mentioned in the subject, with “and” as the link), the verb must be plural. If the subject is “one” (as in the example above), the verb must be in the singular. By its very nature, “both” will be a modifier to a plural subject, so the sentence will take a plural verb.

Here is another example: “My Dad and I both own a car” means that it is one car, and it is jointly owned by the two of us. “My Dad and I each own a car” means that there are two cars, and one is owned exclusively by my Dad, and I own the other one by myself. And, as a final note, be sure that when you use “both,” that you refer to two actors, and two actors only. “Both” is binary; “all of them” or “all” is used for more than two.

“between” and “among” — Like “both,” “between” only works with two nouns or pronouns. (The “tween” part is an archaic reference to “two.”) If you are referring to more than two, then use “among.” For example, the common expression, “Between you and me and the gatepost . . . ” (also the title of a book by Pat Boone) is technically incorrect, as three entities are privy to the secret, not just two. (And note that the “you” should be in second position, not first; “me” should be in third position, not second.) Finally, when you use “between,” and you are one of the parties, please say “between you and me.” “Between” is a preposition; it takes the objective case. “Between you and I” is not only stuffy; it is wrong, too.
“borrow” vs. “loan” — First, the verb is “lend,” not “loan.” “Loan” is a noun. So “Loan me that book” is a barbarism. It should be “Lend me that book.” Secondly, “borrow” is what the person does who receives the item; “lend” is what the person does who hands it over. Incredibly, there are people in this country that say “Borrow me that book, will you?” That is probably a very “back woods” dialect in action.
“nauseated” vs. “nauseous” — To be sick at your stomach is to be “nauseated.” If you are “nauseous,” you are making other people sick to their stomachs! “Nausea” is the condition of digestive illness, of dyspepsia. Something that creates the problem is “nauseous” or “nauseating.”
“different than” vs. “different from” — “Different than” is wrong. The “than” sneaks in because it is almost always used in comparisons: “Kathy is shorter than Sally.” But “different” is not a comparative adjective (like “shorter” or “more”). It is an absolute. Distinct nouns “differ from” one another, not “differ than” one another. If you substituted “distinct” for “different,” you probably would not make the mistake. No one says, “This one is distinct than this other one”; so why would you say “different than this other one”? This is an example of how the “sounds OK” school of grammar can lead you astray. Some argue for an exception if “than” does not merely connect the “different” to a noun or pronoun, but rather introduces a clause as a subordinating conjunction: “This food tastes different than it used to.” This is a bogus exception: the syntax is correct for a comparative, but not an absolute adjective such as “different” or “distinct.”
“decimate” — This word means to kill one out of ten of a vanquished enemy’s soldiers. This was the Roman practice, an effective act of terror to make the other prisoners of war compliant. It derives from “decem,” the Latin word for “ten.” So why would you use this term to mean “obliterate” or “wipe out”? The only answer is ignorance. Now you know. Say “obliterate” or “annihilate” when the slaughter is widespread.

“only” confusion — Consider three sentences: (1) “Only I kissed the bride”; (2) “I only kissed the bride”; (3) “I kissed the bride only.” Can you identify three different meanings, according to the placement of the “only”? In (1) no one else kissed the bride; in (2) I kissed her, but I didn’t do anything else to her; and in (3) I did not kiss anyone other than the bride. Keep your “only” as close as possible to the word you are trying to affect with it; otherwise, you could be conveying a much different message from the one you have in mind.