Just yesterday I was doing a fartlek on a treadmill (snow in the northeast). On my very last "on" I was running 4:40 pace and stepped half on the belt and half on the stationary part in front of the belt. Stumbled but somehow stayed upright and recovered. My heart rate went even faster than it already was as the adrenaline kicked in. Thought for sure I was flying out of the gym window behind me.