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Giant Enemy Crab used Massive Damage.
9999 (or 99999 or 999999999 depending on the maximum HP) damage on everyone.
If anyone is still alive, Giant Enemy Crab continues to spam Massive Damage until:
YOU AND YOUR FRIENDS ARE DEAD. GAME OVER.
Unless you equipped the Ultimate Rare Equip of Rareness, with a 1/1000000 drop chance, then Massive Damage does 0 to that character.
Comments
If you have Reverse status up the game catches in an infinite loop :V
Divide by zero doesn't cause an infinite loop, it throws an exception. Most consoles will yell at you in debug builds for that, but if you don't handle it properly in runtime builds the default catch is to just quietly return zero and hope the fault either blows up quickly or is inconsequential.