The supreme importance of training and calm thinking.
My fiancee and I were scuba-diving off the Spanish coast about a year ago, and got caught by a very strong current off a rocky point. After trying the (futile) exercise of swimming against the current to reach the rocks, I saw her get pulled away. So I let go and rejoined her as she aborted the dive and surfaced (this took place at 80 feet depth). On the surface we got caught in a whirlpool up against a rocky cliff-face. Everything worked out fine, and the divemaster returned to the boat and picked us up. Once we were on the surface we joined with an emergency lanyard, so we wouldn't get separated.
It is however frightening to realize what could have gone wrong if we had been less experienced or poorly trained. We could have used up our air trying to swim against the current. We could have become exhausted from swimming against the current, started hyperventilating and passed out. We could have ascended too quickly, risking injury or death from lung overexpansion. We could have panicked (which is often what kills divers). Luckily we were well trained and experienced, so we were fine.
My point - training and calm thinking is what is needed is stressful situations. I have found it to be true in both sky diving and ocean diving, and I think it is true when it comes to running around a house or apartment in the dark with a gun.