When you have an item in your hand that can be combined with another item (e.g. a stick and knife), and you make use of the quickbar function (holding the number of the item you want to combine that's in the hotbar, e.g. number 6 for the knife), and perform the combine action (holding left mouse), makes you unable to interact with items and objects. Your hands will be seen as empty, and dragging items from inventory to the hands, or using a hotbar key will allow you to use those items normally.

To rid of this, you need to hold the hotbarkey of the corresponding object you used.

Note: I have not tested if this issue persists when the item in the hotbar 'despawns' (e.g. when you use all the ammo if that was in your hotbar).