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 is for the version of mc your launching. eg 1.6 mods for mc 1.6, 17 mods for mc 1.7

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, which only forge=, liteloader & mods that specifically specify to be put in the mc jar.

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

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