Every day is checked before it ships: it has to be hard, and it has to be solvable more than one way.
Play today's puzzleA day only reaches the calendar if a search over hundreds of plans finds at least five genuinely different ones that work, and if the best of them still fails often enough to be worth three attempts. On a typical day nine distinct plan families work, the best of them succeeds about 55% of the time, and sending the team out unchanged succeeds about 10%.
You are not hunting for the one move the designer hid. You are looking for a plan that beats this particular opponent, and there are several. Two people can solve the same day completely differently and both be right, which is most of what makes it worth arguing about afterwards.
When a plan fails, the game tells you which part failed and what the opponent did about it — the duel, the numbers on both sides, and the minute it happened. You are never told you were simply wrong.