Last summer, my spouse left behind his hipbelt, which he realized only in Yosemite valley the night before a two-week trip (think bear cans). This is when we really appreciated the valley shops.
Couple years prior, I lost my sunglasses on day 1 of a five-day trip in the Sierra, in the winter, with beautiful cloudless skies most of the time. Now I always have eyewear retainers.