java.lang.AbstractMethodError (Mod or API Version Mismatch)

What the log looks like

java\.lang\.AbstractMethodError
java\.lang\.AbstractMethodError: Receiver class .+ does not define or inherit an implementation

How to fix it

1. Locate the `java.lang.AbstractMethodError` line and the stack trace lines directly underneath it in your log to see which mod, plugin, or API package (for example, `org.spongepowered` or a mod's package name) triggered the call.
2. Verify that the offending mod or plugin matches your exact Minecraft version and server/loader API version—for example, ensure an API 7 Sponge plugin is not running on an API 8 server, or a Forge 1.20.1 addon is not running on 1.20.4.
3. Update the crashing mod, its required library/API mods, and your mod loader or server build (such as SpongeVanilla, SpongeForge, or NeoForge) to their latest stable builds, since beta builds occasionally ship with missing Mixin method implementations.
4. If the crash started right after updating a core library mod (like JEI) or your mod loader, downgrade that library/loader to the previous version or disable the dependent addon until its author releases an updated build compiled against the new API.

Check your own log