The bug

When you press ESC while you are changing settings for a world you want to generate (for example "Superflat" or "Customized") the game returns to the main menu instead of the world list.

How to reproduce

1. Open the "Superflat" settings for creating a new world
2. Press ESC
   → You will be returned to the main menu instead of the world list

The reason

The following is based on decompiled version of Minecraft 1.8 using MCP. All method and class names are the names used in the decompiled version.

The reason why this happens is because both net.minecraft.client.gui.GuiCreateFlatWorld and net.minecraft.client.gui.GuiFlatPresets call the protected void keyTyped(char typedChar, int keyCode) throws IOException method of the net.minecraft.client.gui.GuiScreen class which returns to the main menu if the Escape key is pressed. In contrast to this the net.minecraft.client.gui.GuiCreateWorld class does not call this method and binds Enter and numpad Enter to create the world. A default binding for the Enter keys might make sense.