Plates (46, 80) mounted on support structure (22) of a machine for packaging articles in sleeve-type carriers act as guides for the moving articles and as detectors of article jams which occur inside open sleeves prior to the closing of the end panel flaps of the sleeves. The plates are mounted for movement toward and away from the moving carrier sleeves and are biased (48, 102) inwardly toward the path of movement of the carrier sleeves. When a jam occurs the pressure of articles moving outwardly of the sleeve causes the plate to pivot away from contact with a sensor (74, 96), thereby automatically shutting down the machine. The plate extends to the flap closing area so as to detect jams at any point between the loading station and the flap closing station.