The artillery dice is usually rolled twice when a cannon is fired, so there are two chances of rolling a misfire result. However, the two results will be different. If a misfire result is rolled on the first dice, the cannon has literally misfired and may explode. If a misfire is rolled on the second roll, the 'bounce' roll, the cannon is unharmed, the misfire result merely indicates that the cannonball has stuck in the ground where it hits and travels no further. If the shot lands on top of a model then that particular model is hit as normal, but there is no further bounce damage.
If a misfire is rolled as the cannon is shot, roll a dice and consult the misfire chart.