I tried an unsolvable pattern as often encounted in minesweeper:
https://minesweeper.kariya.cc/play#MTt2YW5pbGxhO3ZhbmlsbGEsbWluZU51bWJlcjsxLi4xOjIuLjI6ITIyITsxIT8xOjIhPzI6ITIyIQo=
This app showed a mesage of ‘cannot import game. invalid or unsolvable’ and detected correctly.