Using Forge-1.12.2-14.23.5.2795 and Latest Versions of Quark, Quark Oddities and AutoRegLib.

When unequiping the backpack into your inventory it creates a ghost Item and forces you to close out of your inventory in which the real Item will be tossed onto the ground, happens for normal and unbalanced mode.

Here is a video - https://www.youtube.com/watch?v=PvYUyMKZq6w&feature=youtu.be
(Just a side note the video was recorded in a dev enviorment but I have tested it in the actual vanilla client and it produces the same effect)