For us problem #1 are couples who are on multi-stop trips and have a lot of their own things. Our running theory is that one partner packs and the other helps cleans up and mistakes whose things are whose. We lost an entire set of forks and some small hand towels that didn't match the rest of our towels. We might resort to labeling or buying things with unique patterns/colors.
Problem #2 are children and teens. We've lost remote controls, game pieces, had pea gravel end up in the hot tub, water hoses left dripping, and RV septic system clogs. We bat over 900 on adult only stays, but about 500 on stays with kids.