I just finished cube 73737 of the Mystery cell (3?, 4?, d?). It worked fine, and had extra - a piece of a neuron outside the cube, colored gold, that rotated and moved with the cube.
The gold-colored segments are part of a bug that usually occurs after extended playing. While visually bothersome, you can just ignore them. Since the gold segment is outside of the active cube space, it has no effect on the traced segments you submit.
Sometimes the gold segment bug results in fun images such as this llama!