Spelling

Wrong (sometimes because it imitates French spelling)	Right
Independant	Independent
Unvisible	Invisible
An unique...	A unique (You may think that it should be "An" because the next word starts with a vowel, but remember that this "u" is pronounced "you," therefore it sounds as if it started with a consonant.)
A hour later	An hour later (Again, the fact that the word starts with an "h" does not matter. What's important is that in this word, the "h" is silent, so the word sounds like it starts with a vowel.)
Totaly	Totally (two Ls)
Dictionnary	Dictionary (only one N)
Priviledge	Privilege
Transfered Transfering Dimensionned Dimensionning Positionned Positionning Developped	Transferred Transferring Dimensioned Dimensioning Positioned Positioning Developed
Note: Looking at the above list, you may wonder when to duplicate the verb's final letter ("transfer" becomes "transferring") and when not to do so ("position" becomes "positioning"). If you have some intuition about English pronunciation, you can understand why these differ. In the first case, the consonant has to be duplicated because otherwise the pronunciation would change: "transfered" would sound like "trans-fear-d", not "trans-fur-d."

More misspellings:

Wrong Right

Usefull Useful (one L)

Theorical Theoretical

Matrixes Matrices

Indexes Indices

The solution was choosen The solution was chosen

We seeked a solution We sought a solution

Wrong	Right
Usefull	Useful (one L)
Theorical	Theoretical
Matrixes	Matrices
Indexes	Indices
The solution was choosen	The solution was chosen
We seeked a solution	We sought a solution

Near-Homonyms

Some people have trouble keeping the pronunciation of words like "rip" and "reap" separate, because in other languages there may not be the same strong distinction between short and long vowels. But some people seem to actually confuse the meaning of these "near-homonyms," as shown below:

Wrong Right

We will rip millions of dollars from this deal. We will reap millions of dollars from this deal.

Wrong	Right
We will rip millions of dollars from this deal.	We will reap millions of dollars from this deal.

Possessives of Nouns Ending in "s"

When forming the possessive form (also known as the genitive) of a noun that ends with the letter S, this author continues to hold that only the apostrophe is added at the end, and the "possessive s" is omitted. For instance, it is "Gauss' theorem," not "Gauss's theorem." It turns out that in Strunk & White's "Elements of Style," which is often considered the ultimate concise reference for technical writers, the authors recommend to write "Gauss's," "Charles's," etc. I do not recommend this, because it looks and sounds ugly. There!

American vs. British Spelling

"When in Rome, do as the Romans do": there are differences between the American English and British English spellings of some words, and they are not all as obvious as the famous "color" vs. "colour" or "organization" vs. "organisation." For instance, the past forms of the verb to learn are learnt if you follow the British English spelling and grammar, but American English has switched to learned. The following guidelines will help you decide which form to use:

It is considered normal for a British or American writer to stick to the version of the language he or she has learned. However, as a non-native English speaker, if you write for a British or an American audience, you are facing a greater expectation to conform to the local rules or to the habits of your readers (OK, it is unfair, but so what?).
In general, "Schlumberger English" follows the American spelling unless the writer is British himself/herself.
If you are neither American or English, and you are writing for a mixed audience, then at least you need to be consistent thoughout. You cannot say "at this point, minimize the colour map!"

Currency

Currency amounts are not spelled in the same way in English and other languages... and there are even some differences between American and British English. The main things to remember are that (a) the dollar sign always comes first, (b) the separator for groups of three digits is the comma, while decimals are preceded by a period, and the exponent (e.g., "million") comes at the end. Here are some American English examples:
    $5.17
    $50
    $500
    $5,000 or $5K
    $5,000,000 or $5 million or $5M
In this list, you may also notice that there is no space between the dollar sign and the first digit, and that there is a space after the numbers if the exponent is spelled out ("million," "billion," etc.) but not if it is abbreviated (K, M, B).

In British English, the words "thousand," "million" and "billion" are normally abbreviated "k," "m" and "bn" instead of "K," "M" and "B." In traditional publications (books and newspapers), the decimal point is supposed to be half-way up the line, not on the character baseline. This last practice has fallen into disuse with the advent of e-mail and word processing.