As in title, Firing at a ship (or ship cores) with a 99.X effect weapon (IE, 99.8% effect as in this blueprint: {F4915})
1) Spawn the attached ship, ore, create a ship that has , eg, 1000 cannon, 1000 cannon slave, and 998 stop effect module
2) spawn in 2 ship cores, attach a reactor module to both, and attach a shield regen block to only one
3) fire upon the shielded core - notice the 0 damage popup, yet draining shields due to damage
4) Fire upon the core with just the reactor, notice 0 damage again, but this time no block damage and 0 damage popup. This is despite the fact that the weapon is actually dealing 5 damage per shot in the case of the BP