There was no crash, it's just that in vanilla (no mods or mod loaders) a splash potion with Instant Health at amplifier/potency 125 would kill anything that is splashed with it, but with forge installed it does absolutely nothing The command for the potion is: /give @p splash_potion{CustomPotionEffects:[{Id:6b,Amplifier:125b,Duration:1}]} Without Forge - It will kill any entity/player that you splash with it, even players in creative mode With Forge - It does nothing