FAQ

Minecraft not launching

There are many issues possible for this, here are some issues and solutions.

Mods are a different version

Makes sure that all the mods for the instance you are launching are for the version of Minecraft you're launching -- eg. 1.6.4 mods for Minecraft 1.6.4, 1.7.10 mods for Minecraft 1.7.10 and so on.

Putting forge/LiteLoader mods in jar mods instead of loader mods

Whether you're loading forge mods or LiteLoader mods, they all need to be added via. loader mods. Jar mods are added to the Minecraft jar, and that is a rare occurrence with most recent mods.

Why am I getting weird crashes and/or slow rendering with a NVidia Optimus graphics card on Windows?

There's some black magic going on here, try renaming MultiMC.exe to Minecraft.exe.