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 mispronunciation 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.