Even if you're not a Boy Scout you should always follow the Scout motto of “Be Prepared” on your hikes, backpacking trips, camping, or outings. Always have the proper gear and everything you need to survive the day or however long your trip will be plus any extra items you may need if something goes wrong or your trip is extended for any reason.
Have an extra meal, or just a little more water or a water filter. Take extra clothes or rain gear if the weather should turn bad. It may add a little bulk and weight to your pack but it could save your life. A rain jacket doesn't just protect you from the rain it can also protect you from the wind and add a layer of warmth.
A good example is a recent trip we did to the Eastern Sierras in California. The forecast said it would be between the high 70’s in the day and dip down into the 30’s at night with a slight chance of rain for one of the days. But in the Sierras the weather can change hour by hour and it did. With a day hike planned to the Palisades Glacier the sky’s wear blue with a few clouds here and there. Once we made it past the Sam Mack Meadow and almost to the ridge above the Glacier the weather had shifted into a full on blizzard with white out conditions. Not all of us in the group were prepared for this type of weather, but between all of us who were prepared we shared gear to make it down the mountain safely.
If the situation gets worse or someone gets hurt, it's always good to have some extra items on hand to help you stay the night, or as long as it takes to get out or help.
Basic Emergency kit
Sandwich Bag - Primary function is to keep all the below items organized and dry, but the bag can also be used to gather water if need be.
Emergency Blanket - Used to keep warm or dry from the rain
Toilet Paper - No instructions needed. But, it can also be used for fire starting
Small Zip Ties - Can be used to attached or repair items
Safety Pins - Used to mend clothing or lance blisters
Guyline - Can be used to attached branches for a shelter, or attaching items.
Small knife with scissors - cut stuff
Oatmeal Packet - Food
Needle and Thread - Repair clothing or as sutures
Duct Tape - Many uses, like repairing your shoes, starting a fire
Basic First Aid items - Wound care / Pain relief. You may want to include any meds you need.
Matches - Fire
Items can be replaced or added as you need. The above list is just to get you started.