this only happens with mystcraft installed but its is saying it is a forge bug type thing
java.lang.NoSuchMethodError: net.minecraftforge.common.DimensionManager.saveDimensionDataMap()Lan;
at net.minecraftforge.common.ForgeDummyContainer.getDataForWriting(ForgeDummyContainer.java:66)
at cpw.mods.fml.common.FMLCommonHandler.handleWorldDataSave(FMLCommonHandler.java:409)
at aeb.a(SaveHandler.java:215)
at net.minecraft.client.Minecraft.a(Minecraft.java:1843)
at aoo.a(GuiCreateWorld.java:266)
at apn.a(SourceFile:64)
at aoo.a(GuiCreateWorld.java:405)
at apn.d(SourceFile:112)
at apn.m(SourceFile:100)
at net.minecraft.client.Minecraft.l(Minecraft.java:1495)
at net.minecraft.client.Minecraft.J(Minecraft.java:845)
at net.minecraft.client.Minecraft.run(Minecraft.java:775)
at java.lang.Thread.run(Unknown Source)