Dispensers do not receive redstone signals from a powered block that is adjacent to it when in a series. Attached are screenshots of examples with three dispensers in a row (though this bug has been found with as few as two in a row). Clicking the button three times (for example) should deplete the items inside each dispenser three times. However, when this experiment is performed, the number of items depleted is not accurate. In fact, after the first time the button is pressed, two of the three dispensers will be ignored with each subsequent press. This does not occur when only a single dispenser is present.