The instructions for this excellent puzzle state:
Each of the across clues contains a misprint of one letter, always occurring in the definition part. Answers should be entered correctly spelt. Down clues are normal but their answers are to be entered with a misprint of one letter only in each, always in a checked position. These misprints correspond to the misprints in the across clues, in clue order, i.e. 1 Across/1 Down, 6 Across/2 Down … 33 Across/28 Down. No more than one letter in any across answer forms the misprint in a down entry.