1. simple dud (ie bad fuse, trigger or warhead / factory defect)
2. Magnetic Detonation prematurely (usually in far northern latitudes)
3. Failure to detonate magnetically (if depth set too deep, the magnetic trigger never detects the ship overhead)
3. Torpedo breaches the surface and hits the next wave, triggers the detonator and the torpedo explodes prematurely
4. Torpedo strikes the target at too shallow of an impact angle and the trigger is not engaged as a result (torpedo was good, it was just a bad shot)
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