Negligence is almost by definition a hindsight view. If nothing goes wrong there was probably no negligence, or there was no reason to look for it because there was no damage done. If something goes wrong your every action becomes scrutinized. What you did, when you did it and what you did not do all matter.
Remember, torts 101--duty, breach, causation and damages.
Lets build on that a little and add some more small rules. You have no duty to assist or rescue anyone. However, once you begin or initiate a rescue, you now have a duty to make sure that person is OK until handed off to safety. Safety would be home, a hospital, the police etc. . .
So, lets say you are the instructor of a lesson. Student crashes at fence. Do you have a duty? Yes. You are in charge and giving the directions. But you are not liable unless there is negligence and damages. Student gets up, shakes off dirt, you say nothing, student re-mounts and tries again. More duty? No. But if you attempt a rescue, for example if you step forward to ask if student is OK, then you have a duty to make sure student is actually OK. You have assumed another duty and begun a rescue attempt.
The common law reason behind this rule is that someone else will be discouraged from helping because they believe you have things covered. So if you act like its your duty it becomes your duty.
Student crashes in your lesson and you ask if he is OK and then let him continue and the next day student have massive brain malfunction? Your problem? Yep. You assumed a duty and if you did not see student through to the ER for that CAT scan you might be liable. Remember, all those fancy laws and waivers won't protect you from negligence.
Now you have "what you knew or should have known" plus "what you did or should have done".
Here is an example. Rescue attempt or not? Comments are open.