With me it's tent stakes and sierra club cups.
Not that I go loosing these items all the time, but of over the decades these two items top my list.
For twenty years I packed a 6-1/2 pound Timberline 2 tent, which comes with 8 steel shepherd hook stakes. Now and then I'd leave one sunk in the earth somewhere, and when pitching camp at an established site or cleaning up in the morning, I'd often find an extra steel or aluminum shepherd hook that someone else left behind!
When I finally retired that tent it had nine stakes of all different types, some steel, some aluminum, and one of those twisted steel skewers!
So, I guess over the years I came out ahead of the game...
Now I've also carried a steel Sierra Club cup clipped to my backpack belt for decades.
I found it very handy for drinking from every stream, brook, seep, creek, runnel, rivulet and mud hole I came across. I bought my cup Sierra Club cup back in the early 1980s, or maybe it was in the late 1970's?
Eleven years ago while canoeing the Bowron Lakes up in Canada I leaned over the side of the canoe with it to scoop up a drink and dropped the cup! My wife and I spent twenty minutes retrieving that cup!
We were upstream of a lake on a small tributary at the time. We spun the canoe about and my wife in the bow seat reversed position facing the stern so she could solo paddle like heck and try to keep the canoe in position over the cup on the fast flowing stream. I knelt forward and dug through our huge Duluth packsack for our Timberline tent, pulled out the longest aluminum pole, one of the steel shepherd hook stakes and some duct tape. I fastened the stake to the pole, leaned over the side and went fishing for the cup. The water was ice cold and the cup was visible on the bottom about five feet down.
More than once my wife suggested we just give up, but I was determined! I'd had that cup for a very long time and was not going to leave it. Besides, it was the start of a two week trip and I didn't fancy not having a cup to drink my tea from! I was starting to get desperate and was contemplating simply diving in and swimming for it when I hooked the cup and retrieved it!
Sadly, I lost that cup again in 2011 on the Long Canyon Loop in Idaho. The snow pack was unusually heavy that year ( about 150% ) and we were doing the loop to durn early in the year. All the stream crossings were a raging torrent and looking back on that trip we should have simply given up. But, I'm nothing if not a thick headed Dutchman, so we pressed on. During one stream crossing my faithful old Sierra club cup was washed away.
I spent some time searching for it hoping it had lodged somewhere on the streambed, but never found it.
So, if anybody finds a Sierra Club cup on the Long Canyon Loop, let me know!
Interestingly, as a result of the influence of y'all lightweight nut jobs, I switched to a lighter plastic cup carried inside my backpack for a spell. But, just this year I've reverted to the heavy old steel Sierra Club cup clipped to my belt. Old habits die hard, it seems.