If you have a large stack of items (say rifle ammunition) in a car trunk, taking a subset of them (say 30 rounds from 300) drops the remainder that you didn’t pick up onto the exterior tile of the container, not back into the container itself. If you don’t then pick them up and put them back in manually, you will drive away without them.
It’s quite frustrating to lose over 500 rounds of .556 NATO bullets in the middle of a massive city.