Read this first: fixing common launcher crashes

Before you open a new thread, check if your problem is on this list. Most crashes fall into one of these.

1. "Wrong Java version for this Minecraft version"
Every Minecraft version needs a certain Java. Old versions (1.16.5 and older) run on Java 8, newer ones need newer Java. The launcher includes the right Java for you, so if you see this, you probably set your own Java path in the settings. Clear it and let the launcher choose.

2. Forge 1.12.2 closes right after the Mojang screen
Many 1.12.2 mods need a Mixin loader. Install MixinBooter into your mods folder and try again.

3. "A mod stopped the game during start-up"
One of your mods doesn't match your Minecraft or Forge version. Remove half of your mods and start again. If it works, the bad mod is in the half you removed. Repeat until you find it.

4. The game starts but is very slow
Turn on CraftBoost in the launcher, close other programs, and don't give Minecraft more than half of your RAM.

Still crashing? Open a new thread with:
- Your launcher version (bottom of the launcher)
- Minecraft version and loader (Forge, Fabric, Quilt, NeoForge, OptiFine)
- Your mod list
- The error the launcher shows you, pasted inside

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