Got a weird crash. My theory: forge version or a real bug. Or something else, because it would've told me about forge version.
Which version of forge does it (mod) require? Did this run on 2772 version. There's no strict description on the version.

Description: Initializing game

java.lang.NoClassDefFoundError: net/minecraftforge/event/entity/living/PotionEvent$PotionApplicableEvent
at java.lang.Class.getDeclaredConstructors0(Native Method)
at java.lang.Class.privateGetDeclaredConstructors(Class.java:2663)
at java.lang.Class.getConstructor0(Class.java:3067)
at java.lang.Class.getDeclaredConstructor(Class.java:2170)
at net.minecraftforge.fml.relauncher.ReflectionHelper.findConstructor(ReflectionHelper.java:255)
at net.minecraftforge.fml.common.registry.EntityEntryBuilder$ConstructorFactory.(EntityEntryBuilder.java:296)
at net.minecraftforge.fml.common.registry.EntityEntryBuilder$1.(EntityEntryBuilder.java:264)
at net.minecraftforge.fml.common.registry.EntityEntryBuilder.build(EntityEntryBuilder.java:264)
at com.bewitchment.common.entity.ModEntities.registerWithEgg(ModEntities.java:85)
at com.bewitchment.common.entity.ModEntities.(ModEntities.java:64)
at com.bewitchment.common.entity.ModEntities$RegistrationHandler.onEvent(ModEntities.java:123)
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_143_RegistrationHandler_onEvent_Register.invoke(.dynamic)
at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90)
at net.minecraftforge.fml.common.eventhandler.EventBus$1.invoke(EventBus.java:144)
at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:182)
at net.minecraftforge.registries.GameData.fireRegistryEvents(GameData.java:787)
at net.minecraftforge.fml.common.Loader.preinitializeMods(Loader.java:628)
at net.minecraftforge.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:252)
at net.minecraft.client.Minecraft.init(Minecraft.java:466)
at net.minecraft.client.Minecraft.run(Minecraft.java:3930)
at net.minecraft.client.main.Main.main(SourceFile:123)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
Caused by: java.lang.ClassNotFoundException: net.minecraftforge.event.entity.living.PotionEvent$PotionApplicableEvent
at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 27 more
Caused by: java.lang.NullPointerException

Needs a later forge version, but this won't be an issue in the rewrite.

That was surprisingly fast.

Thanks. I guess I'll wait for the rewrite (altrough, not sure when this will come out).