In this document, we refer to spelling errors as those errors that are due to the user's ignorance of how a given word is spelt; we therefore leave aside on purpose all possible mis-typings that are just due to an erroneous use of the keyboard.
Since Italian is a fairly phonetic language, there are few possibilities for a standardly educated person to make spelling errors; such errors are not, however, non-existent. We shall list them as follows:
1) Errors in doubling of a consonant;
2) Errors in inserting or deleting an i or an h in certain morphological constructions;
3) Errors due to affixation (prefix or suffix);
4) Errors due to c/q confusion.
In detail, these errors can be described as follows:
1) Errors in doubling of a consonant.
These refer to words that have, for some speakers, a double consonant sound, although there is only one consonant. For instance, a word like adorabile can be mis-spelt as *adorabbile, especially if under some regional pronunciation influence. Other examples include words like rubrica `index-book', where the br letter pair makes it more difficult for some to pronounce it differently from a word like fabbrica `factory'. In this case, too, regional pronunciation has a heavy influence on the error typology.
Conversely, certain words that are written with a double consonant might be mis-spelt with only one, such as when esterrefatto `appalled' is written as *esterefatto. However, according to the literature we encountered, this second pattern occurs in fewer cases than the previous one.
2) Errors in inserting or deleting an i or an h in certain morphological constructions.
Traditional grammar teaches us that if a feminine noun ends in -cia/-gia, its plural form will end with -ce/-ge if the c or g is preceded by another consonant, for instance, spiaggia `beach', provincia `province', etc. When the c or g is preceded by a vowel, the i is kept in the plural form, e.g. bugia `lie', farmacia `pharmacy'. Some nouns are therefore likely to be misspelt in their plural form, either with the extra i (e.g., *buccie, plural of buccia `peel' ) or with a missing i (e.g., *valige, plural of valigia `luggage').
Something similar occurs for words that require the insertion on an h before the plural mark, such as for fungo / funghi `mushroom(s)', however, since not all nouns with similar ending behave in the same way, some mis-spellings may occur (e.g. sociologo / sociologi `sociologist(s)', sindaco / sindaci `mayor(s)', etc.).
3) Errors due to affixation (prefix or suffix).
In compounding a word, the resulting word is not just the addition of a prefix or a suffix: certain modifications can (and do) intervene. For instance, the word sopraffare `to overtake' is composed of the words sopra + fare, but in compounding the f is doubled; the same applies to frapporre `interpose' (fra + porre). However, this is not a hard and fast rule, and doubling does not occur in all cases. Firstly, it never occurs if the second word starts with a vowel (e.g., fraintendere `to misunderstand'). Secondly, it seems that not all prefixes require the first consonant to double: note that tramandare `to transmit' which is composed with tra + mandare, does not double the m. So the real problem here is that people get confused about which prefixes require doubling and which ones do not, and erroneous forms like *sopratassa `surtax, surcharge' as well as *trammutare `to transform' can be created.
4) Errors due to c/q confusion.
Some words in Italian are spelt with the letter combination cqu, but others (although with a very similar sound) do not (that is, either just qu or just cu), and this makes it difficult to remember which is spelt which way. For instance, aquila `eagle' and acqua `water' follow different patterns of spelling, but the sound k is the same (other examples: scuola `school', nacque `[he] was born', acquisto `purchase' vs. equivalente `equivalent', sequestro `seizure', comunque `however', vs. sicuro `safe', `sure', discutere `discuss', lacuna `gap', `blank', etc.).
 We did not encounter examples of mis-spellings like *fraiintendere, but we mention this case as something that a unsophisticated spelling engine might do.