They made a 15 puzzle where instead of numbers the tiles had letters on them, and the goal was to make a particular sentence. That sentence had 13 letters that appeared once and one letter that appeared twice.
That letter that appeared twice was the first letter of the sentence, and its other appearance was at or near the end. Let's call the two tiles with the same letter T1 and T2, where T1 is the tile that is the tile that is first.
They would show someone the solved puzzle, then scramble it, and challenge the person to solve it. When they were scrambling it they made sure to move T2 to the first position and T1 to somewhere near the end.
Nearly everyone who tried to solve it would see the right letter at the first position and never move it, because most people try to solve left to right and/or top to bottom.
On the 15 puzzle exactly half of the possible permutations of the 15 tiles are reachable from a given starting position. Exchanging two tiles in a reachable permutation produces an unreachable permutation.
That means that the right sentence but with T1 and T2 swapped is not reachable and so people who left T2 in front were doomed to failure.