People never plan to get lost or stuck during a day hike, but sometimes it happens. When it does, there's a good chance they'll spend at least one night outside. How miserable they are, and possibly whether they live to see the morning, will partly depend on the gear they brought along.
If you carry the Ten Essentials, you're properly prepared in terms of gear. Also, I've described kits for unintended overnight stays, here and here, but the bulk and weight of those kits may discourage people from carrying them. Here I assume that you're not yet convinced of your mortality and want to carry as little gear as possible; I describe what I consider the absolute minimum gear for an unplanned overnight stay. The good news: that gear weighs six to eight ounces and fits in a quart zip bag.
If you're stranded outside overnight, your number one concern should be staying out of the wind and rain. Either can suck the life-giving heat out of you, and in combination they're a proven killer. The survival blanket shown above will block both wind and rain, weighs 3 ounces, and fits in a jacket pocket. Assuming your clothing is sensible for the daytime conditions, the survival blanket should get you through the night. When you're stranded you may or may not be able to start a fire, but you'll always be able to use the survival blanket.
A fire doesn't keep off the wind and rain, but it will help keep you warm (especially if your clothing is less sensible than you think). Or, if your clothes are wet, you can dry them by the fire. If someone is looking for you that night, they're more likely to find you if you're by a fire. Two items shown above greatly increase your chances of actually starting one. The Bic mini-lighter is small but will provide a more reliable flame source than matches. The foil-wrapped item is a Lightning Nugget brand fire starter, but that's just what I use; the critical thing is to have something that burns until the wood you gathered catches fire.
You won't die of thirst overnight, but if you get lost or stranded you're likely to run out of water. Even in New Mexico you may come across water in the wild, but if you do you need a way to purify it. The packet of four chlorine dioxide tablets shown in the picture weighs less than a tenth of an ounce, and will let you purify a gallon of water. The down side of the tablets is, to protect against Giardia you'll need to let the water sit for four hours before drinking it. A better option is the Sawyer Mini filter and half-liter squeeze pouch also shown in the picture. That adds three ounces to the overnight kit but allows you to drink all the water you wish, as soon as you find it.
It's also good to have a source of light. If you're cutting corners, the keychain light shown in the picture puts out 16 lumens. And yes, the whistle is essential. If you're yelling for help you'll go quickly go hoarse, but as long as you can breathe normally you can keep whistling.
The second pic shows all of the items in a quart zip bag. If you go with the Sawyer Mini as your water purification option, this "oops" kit weighs eight ounces. If you go with the four chlorine dioxide pills, the kit weighs under six ounces. If you can't be talked into careful adherence to the Ten Essentials, please consider taking a quart zip bag with the items I've discussed.
P.S. I haven't included any food in this minimal kit, for the simple reason that people don't starve to death overnight or even over a few days. It would be easy to add an energy bar, peanuts, or something similar to this kit and make your unplanned overnight stay that much less miserable.