If you drop an item onto a characters portrait at the bottom of the screen instead of onto the correct slot on the left when in the inventory screen, the item will disappear until another item is dropped onto the same portrait. The second item will then disappear, until the process is repeated, meaning that at least one item is ‘lost’.
The item isn’t lost – it’s placed in the inventory underneath the portrait, as if the portrait weren’t there. You can get to it by scrolling down the inventory list or using the sort function. Still something that needs to be fixed, of course :)