It walked fine for several minutes. As I was trying out the bottom row of buttons on the remote, its limbs and head flipped around and head spun 360deg (not quite enough to put tension on the servo wire) so I grabbed it and powered it off. After powering on again, it walked fine. But then shortly after joint 12 (I misspoke when I said 13 -- it was 12.) seemed to stop working, because when it tried to walk, it fell to that side and rolled onto its back. I tried a few more resets before I realized what was causing it. The servo seized completely; I can't even spin it by hand.
I replaced it with an SG90 but I will order another MG92B or D. Since the servo seized, I doubt the problem is the brush, but I will try to troubleshoot by disassembling it when I have time. I will update you as I progress.