This happened to me in the original site. I think it happens when the white and black balls collide at the exact same spot of the border.

I think it's just a tunneling bug that happens when the point that make up the wall get messed up a enough. Almost never happens anymore in the v2[1] that I added. This one also allows you to see these points.

[1] https://francisduvivier.github.io/eternal-struggle-with-spee...