All good points, Kurt. Based on recent observations, several more could be added--most of which apply year-around. For example:
Learn about the ten essentials
, and the meaning of the word "essential." Day hikers should pay particular attention to the ones on extra clothing and emergency shelter. If you ignore some of Kurt's advice, or are just unlucky, you may be spending an unexpected cold night out.
If your group wants to split up, have a good reason for it and have a plan. Know where the others are. Stay within visual/FRS/other contact, and agree to reconnect at a particular time and/or place. Hint: "My wife is back up there somewhere. She got sick so I'll wait for her at the Portal" is not a good reason.
A fellow once asked me if rangers sweep the trail from bottom to top at the end of each day, looking for people in trouble. They do not. (The fellow thought they should.)
If SAR is needed, it will probably not arrive for at least 24 hours--after the request gets out. Helicopters are not always available. A stretcher ride for miles down a bumpy trail is not Magic Mountain.