You lose an item when 'auto swapping' when there is not enough room in the inventory.

Best way to explain is with an example.

Make sure your inventory is 100% full and there is one 10 bullet magazine in your inventory (1 slot)

Have a M4 with a 30 or 60 bullet magazine in it (requires two slots).

Now press f and click to load the other magazines (in the game this happens a lot accidentally when trying to open a door or something).

Now the game will try to swap the 60 bullet magazine with the 10 bullet magazine in you inventory however it fails in doing so because by swapping it has one slot free but the 60 bullet magazine requires 2 slots.

What then happens is that you lose the magazine that he tries to put in your inventory. It does not give a message that it can't swap, it also does not drop the magazine on the floor you simply lose the magazine into thin air.

Happend to me twice so far. Second time I notice what was the problem (not enough empty slots).

In my opinion a problem with priority as it means you lose items.

I think the same can happen with other items but the magazine one will be the most common example.