I didn't quite finish the level select or the final area of the game. There was a music track for that area that never got implemented so as an Easter egg, the only functioning buttons are 13-15 that play the track intended for those levels, but I can definitely see how it would be seen as a bug. Did you mean the whole menu as in the title screen too or did you not realize yet that level select doesn't work yet?
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Huh... That definitely would be an unintended side effect and would be a bug if you couldn't interact at all with the menu anymore. Attempting to replicate the bug yielded nothing as I was still able to interact with it, but now I'm paranoid. I added the functioning level select in a post jam update so hopefully something like that won't occur anymore, but unfortunately you never know what these kinds of things.