Look at YouTube videos and there are lots of videos where there was some malfunction of a mortar round.  The person would drop the mortar into the tube no problem, however instead of flying out of the tube at high velocity it would shoot out a few feet up and then drop right in front of the mortar crew.  The ones you see on YouTube must have been only training mortars because even though the crew all run like hell away after it fall just a few feet in front of them, the mortar does not explode.  There was one video that the mortar exploded in the tube but again must have been a training mortar because if it was real one, everyone around the explosion would have died like this Afghan army accident.