I found a puzzle with multiple solutions: https://onlinenonograms.com/en/id/110565 Solution 1 is the expected solution that shows the NONO (heart) (heart) GRAM image. To get solution 2 you delete the black squares in 1st row 15th column and 9th row 14th column and you add black squares in 1st row 14th column and 9th row 15th column. With such a high puzzle number I would have thought that all the checks would have been in place at the time of its creation to detect and reject this puzzle.