Where do black hole microstates get chaotic?

The D1D5 system is a string theory setup that encodes black hole microstates as a conformal field theory. By studying energy level statistics in this system, researchers found that at infinite N the spectrum looks random and featureless (Poisson-like), but at realistic finite N, mixing between different quantum states triggers level repulsion—the telltale signature of quantum chaos. Non-planar terms that vanish at large N are the culprit.