Minecraft 1.8.9, Minecraft 16w06a, Minecraft 16w07a, Minecraft 16w40a, Minecraft 1.12.1, Minecraft 1.12.2
Preset code offset in superflat customization GUI is not reset, causing text to be pushed too far to the left.
- Open the presets screen of superflat mode
- Click a preset whatever you like;
→ You would see just a part of the code and you can get the full code back by pressing button "◀" or click the left side of the text box.
If the code is just fine, click another preset until you could see the glitch.