What Piece of Extra Safety Gear?

So let's say you are heading on a long day trip to an entirely new objective. You want to bring an extra piece of safety gear for some added insurance in case something goes wrong. This would be something in addition to all your regular safety gear. What do you bring that adds no more than a pound or two to your pack (e.g. stove, bivy, sleeping pad, etc)? Genuinely curious.

down sleeping bag

I like your question but for me the answer depends primarily on the circumstances of the tour. High alpine winter exposure concerns? Summer uncontrolled slide risk? Lost in the woods schwack-fest? Precip in the forecast? Drinking water availability? Cell coverage? Am I solo? What are my partners bringing?
The answer can vary from an axe and/or whippet to a bivy, a stove to a crevasse kit, a shovel, crampons or extra layers. I believe that we often tend to over-simplify what the appropriate safety kit consists of.
A couple of catch-all responses might include a helmet, chemical hand warmers and a bit of closed-cell foam to sit on (I have one in each of my packs in place of the internal frame board).

I keep an ultralight emergency bivy bag in my pack on every outing (a.k.a. Rustle the mylar sack). PLB, too.

To answer your wintertime added-gear question, an MSR Reactor-type stove and a full winter-blend canister can convert a cold, sad time into a pretty good one in a jiffy.

Agreed that it is totally situational dependent.

Jetboil MicroMo 0.8 liter is tiny, 4oz of fuel (nested in the cup) is enough for 2 people for a night's dinner and breakfast plus drinking water.

You can almost always construct a snow cave with a shovel (especially in the typical PNW, less so in a continental snowpack). Thus the tiny emergency bivy could make it easily survivable.

Extra layers are always amazingly valuable; down shorts or 3/4 length pants are pretty epic. Your summer sleeping bag packs tiny as another emergency layer.

Eye protection should never be underestimated.

I sometimes carry a Brooks Range sled, which can be used to self rescue.

Interesting subject, I recently read (and just tried to refind it, and couldn't) an article about testing your overnight prowess with your typical day kit at an area close to the car for when you bail. Strong, subtle reminder.

In a group of 3 or more, it is pretty easy to amass the extra gear. The real tough predicament comes with a group of two, where the "extra" stuff adds up quickly and you end up bringing an overnight kit on every outing.

Andy Dappen had an article in a recent Backcountry mag about making fire for an unplanned overnight. It was intriguing, and I suspect that if you could really keep a fire going all night, it would be a really good thing.

I'm doubtful of my ability to do that, so I think the most important thing is to be able to dig a snow shelter so you can limit your cold exposure to 32 degrees F during the night. Then you need to be able to snuggle and share warmth with your ski partner(s).

Seems like the most important tools for that would be shovels, a tarp (to speed up the digging process, for example to roof over a snow trench), and some combination of pads and/or bivi sack to improve heat sharing inside the snow cave. Seems like a breathable sack would be best, to avoid soaking your clothes as you and your friend(s) huddle inside during the night.

Good topic.