Recently I did the hard 10 levels challenge and I lost an arm and a leg. I left to take a break and when I entered the challenge again I regained them. Basically on level 1 I led the fire robots into the saw blades and got cut myself. I exited the game through escape main menu then took a break. When I came back I pressed challenge and the 10 levels challenge again to find my limbs had regrown. I have visual proof this has happened at 8:56-9:36 in the video I have posted.
https://www.youtube.com/watch?v=IQzbn_LmT_g&feature=youtu.be