Hikermor's list is almost exactly like mine, with minor differences in brands and models maybe.

Yes, I can't count the number of people who I have helped who were hiking with little or nothing. I even carried a foreign woman tourist who was hiking a sandy trail in high heels! With no water, in 110+ degree F temperatures! I actually carried her out a mile and a half on my back because her feet were too torn up to walk.

Nothing can substitute for knowledge and experience. A little preparation goes a very long way.

The rule of threes has been discussed much before, but it is a good rule of thumb. You can survive bad conditions for about 3 hours without shelter, 3 days without water, and 3 weeks without food. So pack with these priorities in mind.

Shelter
Water
Signaling
Fire

Signaling is a top priority along the entire continuum. Once you realize you need help, signal for it.

Food is not a top priority, but it can help physiologically and psychologically. The brain requires a great deal of energy and nutrients to function properly. As is taught to the scouts, once you realize you need help, Stop, Think, Observe, Plan (STOP). But it helps to sit down (once you are safe), drink some water and eat a little. It calms you down, feeds the brain, and helps you not make bad decisions, like running off in a bad direction.

I like ponchos, and I try to find ones that are brightly colored so they can be used for signaling. A shelter for shade or warmth is critical. A good rain suit will work well. You need a wind and water proof shelter to retain heat when it is cold, but a poncho can shade you in the heat as well.

Dressing for the weather and terrain is critical. Do not prepare for the expected temperatures, but for the extremes. That is what backpacks are for. I really like my light weight Smartwool base layer. It is surprisingly warm for the thickness and weight, and paired with a windproof/waterproof layer has kept me warm in temperatures well below freezing. Hat and gloves are also essential. In the heat, protection from the sun is also critical. There are many new styles and materials for this, and most of them work well enough. Long sleeves, pants, wide brimmed hat, and sunscreen. The sun can kill you. I even have a bright yellow umbrella from Sea To Summit that I carry at times, great shade that lets the air flow.

I also add a GPS, just to use the track back feature if needed.