As a school psychologist, I have worked with many children who have trouble with spelling. Common spelling mistakes include:
Using the wrong consonant (e.g., spelling cat as kat)
Using the wrong vowel (e.g., spelling seat as seet)
Leaving out consonants (e.g., spelling kicking as kiking)
Leaving out a vowel (e.g., spelling plain as plan)
Writing only one consonant, when a consonant should be doubled (e.g, spelling butter as buter)
Leaving in an “e” that should be dropped (e.g., spelling riding as rideing)
Leaving out the “silent e” (e.g., spelling kite as kit)
Using ys instead of ies (e.g., cherrys instead of cherries)
Spelling words phonetically when a specific suffix should be used instead (e.g., spelling vacation as vacashin)
Using an “s” instead of a “c” or a “c” instead of an “s” (e.g., absense instead of absence or offence instead of offense)
Forgetting rules like “i before e except after c” (e.g., spelling receive as receive)
The errors above are the most frequent ones I have seen in my career as a school psychologist; however, there are many more types of spelling errors a person can make.