You attack a big piece, miss the smaller one behind
A skewer is a pin in reverse: the more valuable piece sits in front, the smaller one behind. Force the big piece to move, then take the small one. You miss this pattern.
vs game: a bishop check forced the king to move, exposing a rook behind. White missed the rook grab.
vs puzzle: queen and king on the same file, a rook check wins the queen via the skewer.