Have a "second" weapon in hand while upstairs in a building. Frequently if you choose to "eat" something and go through the "putting second weapon over your shoulder animation" the item will disappear through the floor. Occasionally once you have completed your action you can go downstairs to that spot and find your item. Other times, if there is no accessible downstairs, you cannot.

I had this happen recently with a high cap vest in the base in novo. I was standing next to one of the trucks trying to put a jacket on but I couldn't- I thought maybe because I had a vest on already. I figured I might need to take the vest off to put a jacket on... so I took the vest off to do so and it disappeared.