Forge Server Tools · every failure mode · guides
A mixin failed to apply — FATAL
A mod patches a Minecraft method at runtime, and the method it expects isn't shaped the way it expects. Almost always: the mod is built for a different Minecraft version, or two mods are patching the same method and one clobbered the other.
Check the failing mod is built for YOUR exact Minecraft version -- not 'close enough'. 1.20.1 and 1.20.4 are not interchangeable. If the version is right, the owner named in the error conflicts with another mod touching the same code; remove one and retest. Performance and rendering mods are the usual pair.
The error names the mixin's OWNER and its TARGET. The owner is the mod to act on; the target is only where the patch landed.
Easy to blame the target mod, which is usually innocent. A mixin frame appearing in some unrelated trace also does not mean a mixin is at fault.
Update the owning mod first — these break on a version bump of the target. If both are current, check the owner's config for a compatibility toggle; many ship one specifically for this. Load order rarely helps and is worth trying only after the rest.
Search your crash report or latest.log for any of these:
org.spongepowered.asm.mixin. ... throwables. InvalidInjectionException Mixin apply failed CriticalInjectionException
The free edition covers the six most common startup failures, and this is not one of them — it is in the paid tool, which checks all 24 and names the mod jar from the stack trace. The fix above is the same one it prints, so you do not need it to solve this.