NaN-speed glitch

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THE NAN-SPEED GLITCH
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-- What is the NaN-speed glitch? --

The NaN-speed glitch is an issue on certain levels where, when the stage begins, it is beaten instantly. This is because the ball reaches a velocity of NaN in all directions, hence the name "NaN-speed glitch".

-- What causes it? --

It is caused by there being a triangle with zero area in the LZ.

-- How would I fix it? --

If you use Wings, select all objects, right click, and select "Cleanup".
If you use Blender, select all objects, press W, and select "Remove doubles".
Both of these should remove zero-area faces.

-- Can I prevent it from happening? --

Regularly cleanup/remove doubles from your stage.
In Wings, an issue considered to commonly cause it is using Connect to triangulate. Consider triangulating on export instead.