When someone makes a mistake, instead of just showing Sorry! Unfortunately, your sudoku solution was not correct. Please try again! and taking them to a new puzzle, you should point out the error.
When someone completes a puzzle, in addition to Congratulations! You've solved a sudoku puzzle successfully and helped provide a weeks worth of clean water. you should provide some random statistic about clean water in the world and perhaps how well you're doing relative to other players.
The scoring system doesn't work either (or perhaps you were expecting cookies to be enabled without mentioning it)?