[seanalltogether]

Except that your puzzle has multiple solutions, so I'm guessing that's why it breaks the heuristics. The same would be true of a sudoku puzzle with too few starting numbers.

  O O X X O X
  X X O O X O
  O O X O X X
  O X X O O X
  X X O X O O
  X O O X X O

[bumbledraven]

Your solution is not valid. The 3rd column and the last column are duplicates.