Mortar fuzes can be tricky. I remember firing WWII vintage 81 mm rounds with VT fuzes at Schofield Barracks back in ‘77. They were fired with charge packs for max range and the fuzes were setting the rounds off in mid air just a few hundred meters from the firing pits. Had to shut down the range that day...
Nothing like old rounds to keep you on your toes or a misfire in the tube!