Pistol squats: careful there. I damaged the cartilage on the back of my patella doing something similar. The damage was caused merely by stepping up onto an almost waist high platform a couple of times. This is not the usual consult-a-doctor bullshit: I was 23 and deadlifting 275+ for reps, so I wasn't "weak" or out of shape. Make sure you have the proper strength for a full pistol squat before trying it.