Your spelling checker should catch this one, but judging by the popularity of “mispell,” “mispelled,” and “mispelling” on the Web, it slips by many people. These words need two S’s: one to end “mis-” and another to begin “-spell.” So the words are “misspell,” “misspelled,” and “misspelling.” This ranks as an embarrassing spelling mistake right up there with “writting.”