Why did my modded Minecraft crash?

Open the newest file in crash-reports (or logs/latest.log) and find the error. Most modded crashes are one of these four.

Paste your crash report and Modkeel names the mod that most likely caused it.

• Mixin apply failed

  MixinTransformerError, InvalidInjectionException. A mod could not change the game's code: usually made for another Minecraft version, or two mods change the same code.

• NoClassDefFoundError, NoSuchMethodError

  ClassNotFoundException, NoSuchFieldError. A mod used code the game does not have: a missing library mod or a version mismatch.

• Mod loading failed

  Incompatible mods found!, Mod loading has failed. A missing or wrong mod, a mod for another loader, or the same mod twice.

• OutOfMemoryError: Java heap space

  The game used all the memory the launcher gave it: give it more RAM, or remove heavy mods.