The "missing" down clue was surely because there are only 26 letters in the alphabet, not any mistake. In these kinds of puzzle, you need just enough information in the clues to let you solve it, but very little, if any, more. This puzzle seemed really nice to me in that respect. There was always just enough (but not too much) information to figure out the next move.
Interesting to read of people going at this sort of crossnumber by guesswork and backtracking when they reach an error. That's inevitably going to make things very difficult and somewhat misses the point of logical deduction!