Looks like my ridiculously poor hkpListShape structure has come back to haunt me. I forgot that the max number of shape keys you can have is 8, so after reconstructing the shape 8 times, everything fell apart as soon as I tried a shape cast.
Now instead of just replacing the old list shape with a new one containing it plus a hkpTransformShape, I’m properly rebuilding the whole thing and it seems to be working!
Very late to be worrying about this sort of stuff! I wonder how many bugs I’ll find tomorrow?
