Mibibyte is a common misspelling, presumably from the "i" in the abbreviation or confusion about how the surrounding prefixes (i.e. "kibi" and "gibi") were actually formed. The "i" is actually from the end of the prefix. Tebi- and pebi- are similarly abbreviated Ti- and Pi-, but they are not usually misspelled, as storage sizes (and corresponding terms) currently are only this large in specialized situations.