Misspellings occur mainly with homophones and words with similar letters. Homographs and homonyms, obviously, can't be misspelled because one of their features is that they're spelled identically. Misspellings also occur in words with unusual spellings and/or that don't follow the most common "rules" that people learn. Common categories of misspelling include:

Double Instead of Single/Single Instead of Double:

Many people puzzle over the number of c's and m's in accommodate and the number of r's in occurred. Naturally, this confusion is not automatically alleviated when one is keyboarding.

Mistaken Spelling of a Vowel or Consonant Sound:

When we work with a language in which /k/ can be spelled /c/, /k/, /ck/, /q/, /cq/, etc., it's not wonder that sometimes misspellings come from assigning the wrong spelling to a sound. This goes for vowels sounds as well, of course.

Mistaken Spelling of a Suffix (or/er; able/ible):

This is like mistaking the spelling of a vowel or consonant sound, but for a slightly larger word part: bigger than a single vowel or consonant sound, but smaller than a word, these homophonic affixes can give keyboardists trouble.

Substituting a Homophone:

The famous errors of mistaking to, too, two; your, you're, yore; and their, they're, there are common at the keyboard. We're less likely to mistake words that name similar things but look and sound very different (for example: ewe and ram) than we are to mistake these words, which take extra cognitive energy to distinguish.