Most misspellings can be categorized in one of seven groups. Here are some examples for each of those types.
1. Incorrectly Repeated Consonants
In some words, consonants are awarded extraneous twins, such as a doubling of the first t in commitment or of the r in harass (the latter perhaps from confusion with embarrass, in which r is doubled. Other common erroneous doublings including the n in inoculate (perhaps because of innovation and other words in which n is doubled), the s in occasion (many words, like expression, do have a double s), and the c in recommend. Note that in many of these words, there’s already a twin double consonant, which may also confuse writers. (One word that does have two twin consonant pairs, accommodate, is often misspelled with only one m.)
2. Wrong Vowel
Using an incorrect vowel is a common problem, leading to such misspellings as definately (or the bizarre variant definatly), dependant, privelege, rediculous (a heretofore virtually unknown mistake, prompted by emphatic pronunciation of the first syllable, that has gone viral as more people are exposed to it online), and seperate. The correct spellings are definitely, dependent, privilege, ridiculous, and separate.
3. Wrong Consonant
This type of error is less common than those of the vowel variety, but two of the most commonly misspelled words in this category are consensus (in which the first s is replaced with a c) and supersede (in which the second s is replaced with a c).
4. Reversed Order of Double Vowels
Many words with two consecutive vowels, especially those with a pairing of e and i, look odd no matter which order the vowels appear in, so for many writers, it’s a toss-up as to which is correct. These words are all spelled correctly: gauge, niece, pharaoh, receive, weird.
5. Extra Letters
One word that is often given an extra vowel is mischievous, perhaps because it is often mispronounced as if it were spelled mischievious. Some words ending in -ly, such as publicly, are often erroneously given an -ally ending. Judgment and acknowledgment, spelled in British English (and, well into the twentieth century in the United States) with an e after the g, omit the e in American English.
6. Missing Letters
Coolly and woolly are often misspelled with only one l. Incidentally and other words with the -ally ending, in a reversal of the problem commonly seen with misspelling of publicly and the like, are frequently mistakenly spelled with -ly endings. Liaison often lacks its second i, prerogative is sometimes seen without the first r, and rhythm may lack the first h.
7. Confusion with a Similar Word
The most common type of misspelling, perhaps, is that in which the wrong word in a homophonic duo or trio is employed, such as forward in place of foreword or site (or, rarely, sight) instead of cite.