Mixins are how Fabric and Forge mods change the game's own code at load time. When two mods change the same thing, or a mod changes something that no longer exists, the load fails loudly and names the mod that was applying the change — which is not always the mod at fault.
What you are seeing
Mixin apply failedorMixinApplyErrorduring startup.Critical injection failurenaming a method and a class.- The server exits before reaching the world-loading stage.
- It worked yesterday and you updated one mod.
Why it happens
A mixin describes a change to a specific method in the game. If the game version changed and that method moved, the injection has nothing to attach to. If another mod already rewrote the same method, the two changes conflict. Both produce the same error shape, which is why the message names a mod without telling you the whole story.
What to do
- Read which mod the error names first
The line usually contains a mod id in the mixin package path. That is where to start, not necessarily where to finish.
- Check that mod's version against your Minecraft version
The most common cause by far is a mod built for a different game version. It loaded far enough to try, which is why it looks like a conflict.
- Remove the named mod and start again
If it starts, you have your answer. If it fails on a different mod, you have a version mismatch across your whole set rather than one bad mod.
- Look for two mods doing the same job
Two performance mods, two lighting engines, two chunk optimisers. This is the second most common cause and the error rarely says so directly.
Removing a mod deletes its blocks and items from the world. If the failing mod added content, taking it out to fix startup can cost builds. Test on a copy, and if the mod must go, be explicit with your players about what disappears.
A published modpack is a set of versions that were tested together. Updating one mod inside it is how a working pack becomes a mixin error, and reverting that one mod is usually the entire fix.
The server reaches the world-loading stage and prints its «Done» line with no mixin errors above it.