When you mark a field as mined and then open a field you think is not mined but it has a mine then the game is over. In this case you cannot see which safe fields you marked as mined.
to reproduce:
Start a new game and flag as many random fields as you want.
Find and open any field containing a mine to trigger explosion.
In this 'classic' theme, exploded bomb is black, and wrong flagged is transparent bomb (dark grey)
i made them transparent, to see what number is behind them.
When you mark a field as mined and then open a field you think is not mined but it has a mine then the game is over. In this case you cannot see which safe fields you marked as mined. to reproduce: