1.) Fill your inventory, so there's no free slot left.
2.) Take an <b>ADDITIONAL</b> first aid kit into your hand. <b>IT MUST NOT BE IN YOUR INVENTORY!</b>
3.) Make sure, at least one slot is free in the first aid kit.
4.) Find an item on the ground (max. 3x2 slots).
5.) Right click the item on the ground and click "Take ..."
6.) Recognize it landing in the first aid kit.
<b>UPDATE (4):</b>
Still 100% reproducible in version 0.47.124558 (thx to "mr snipes")
<b>UPDATE (3):</b>
Still 100% reproducible in version 0.33.115188
<b>UPDATE (2):</b>
Still 100% reproducible in version 0.33.115106
This also works with items bigger than 1x1.
<b>UPDATE (1):</b>
Still 100% reproducible in version 0.33.114926
See attached images