Yet Another Mixin Crash

[twothe]

[18:16:23] [main/ERROR]: Unable to launch
java.lang.RuntimeException: java.lang.ClassNotFoundException: net.minecraftforge.fml.common.Loader
	at net.minecraftforge.fml.common.launcher.FMLDeobfTweaker.injectIntoClassLoader(FMLDeobfTweaker.java:67) ~[forge-1.12.2-14.23.5.2847.jar:?]
	at net.minecraft.launchwrapper.Launch.launch(Launch.java:115) [launchwrapper-1.12.jar:?]
	at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.12.jar:?]
Caused by: java.lang.ClassNotFoundException: net.minecraftforge.fml.common.Loader
	at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:191) ~[launchwrapper-1.12.jar:?]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_51]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_51]
	at java.lang.Class.forName0(Native Method) ~[?:1.8.0_51]
	at java.lang.Class.forName(Class.java:348) ~[?:1.8.0_51]
	at net.minecraftforge.fml.common.launcher.FMLDeobfTweaker.injectIntoClassLoader(FMLDeobfTweaker.java:56) ~[forge-1.12.2-14.23.5.2847.jar:?]
	... 2 more
Caused by: org.spongepowered.asm.mixin.transformer.throwables.MixinTransformerError: An unexpected critical error was encountered
	at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:521) ~[LagGoggles-FAT-1.12.2-4.9.jar:?]
	at org.spongepowered.asm.mixin.transformer.Proxy.transform(Proxy.java:72) ~[LagGoggles-FAT-1.12.2-4.9.jar:?]
	at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279) ~[launchwrapper-1.12.jar:?]
	at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176) ~[launchwrapper-1.12.jar:?]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_51]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_51]
	at java.lang.Class.forName0(Native Method) ~[?:1.8.0_51]
	at java.lang.Class.forName(Class.java:348) ~[?:1.8.0_51]
	at net.minecraftforge.fml.common.launcher.FMLDeobfTweaker.injectIntoClassLoader(FMLDeobfTweaker.java:56) ~[forge-1.12.2-14.23.5.2847.jar:?]
	... 2 more
Caused by: java.lang.AbstractMethodError: org.dimdev.vanillafix.ModCompatibilityMixinPlugin.preApply(Ljava/lang/String;Lorg/spongepowered/asm/lib/tree/ClassNode;Ljava/lang/String;Lorg/spongepowered/asm/mixin/extensibility/IMixinInfo;)V
	at org.spongepowered.asm.mixin.transformer.MixinInfo.preApply(MixinInfo.java:1212) ~[LagGoggles-FAT-1.12.2-4.9.jar:?]
	at org.spongepowered.asm.mixin.transformer.MixinTargetContext.preApply(MixinTargetContext.java:1126) ~[LagGoggles-FAT-1.12.2-4.9.jar:?]
	at org.spongepowered.asm.mixin.transformer.MixinApplicatorStandard.apply(MixinApplicatorStandard.java:274) ~[LagGoggles-FAT-1.12.2-4.9.jar:?]
	at org.spongepowered.asm.mixin.transformer.TargetClassContext.applyMixins(TargetClassContext.java:353) ~[LagGoggles-FAT-1.12.2-4.9.jar:?]
	at org.spongepowered.asm.mixin.transformer.MixinTransformer.apply(MixinTransformer.java:724) ~[LagGoggles-FAT-1.12.2-4.9.jar:?]
	at org.spongepowered.asm.mixin.transformer.MixinTransformer.applyMixins(MixinTransformer.java:703) ~[LagGoggles-FAT-1.12.2-4.9.jar:?]
	at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:509) ~[LagGoggles-FAT-1.12.2-4.9.jar:?]
	at org.spongepowered.asm.mixin.transformer.Proxy.transform(Proxy.java:72) ~[LagGoggles-FAT-1.12.2-4.9.jar:?]
	at net.minecraft.launchwrapper.LaunchClassLoader.runTransformers(LaunchClassLoader.java:279) ~[launchwrapper-1.12.jar:?]
	at net.minecraft.launchwrapper.LaunchClassLoader.findClass(LaunchClassLoader.java:176) ~[launchwrapper-1.12.jar:?]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_51]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_51]
	at java.lang.Class.forName0(Native Method) ~[?:1.8.0_51]
	at java.lang.Class.forName(Class.java:348) ~[?:1.8.0_51]
	at net.minecraftforge.fml.common.launcher.FMLDeobfTweaker.injectIntoClassLoader(FMLDeobfTweaker.java:56) ~[forge-1.12.2-14.23.5.2847.jar:?]
	... 2 more
[18:16:23] [main/INFO]: [java.lang.ThreadGroup:uncaughtException:1052]: net.minecraftforge.fml.relauncher.FMLSecurityManager$ExitTrappedException
[18:16:23] [main/INFO]: [java.lang.ThreadGroup:uncaughtException:1052]: 	at net.minecraftforge.fml.relauncher.FMLSecurityManager.checkPermission(FMLSecurityManager.java:49)
[18:16:23] [main/INFO]: [java.lang.ThreadGroup:uncaughtException:1052]: 	at java.lang.SecurityManager.checkExit(SecurityManager.java:761)
[18:16:23] [main/INFO]: [java.lang.ThreadGroup:uncaughtException:1052]: 	at java.lang.Runtime.exit(Runtime.java:107)
[18:16:23] [main/INFO]: [java.lang.ThreadGroup:uncaughtException:1052]: 	at java.lang.System.exit(System.java:971)
[18:16:23] [main/INFO]: [java.lang.ThreadGroup:uncaughtException:1052]: 	at net.minecraft.launchwrapper.Launch.launch(Launch.java:138)
[18:16:23] [main/INFO]: [java.lang.ThreadGroup:uncaughtException:1052]: 	at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

[dominic-03]

I have this problem too when I have Better Portals installed. Without it my game starts fine.

[BoogieMonster1O1]

Read the pinned issues (#296 and #265)