Find / place two items that go together such as in my original case,found a syringe and injection vial next to one another on the floor - may work with other items found 'spawned' together, and even placed items (untested).

(possibly) carry a item or weapon in hand that cannot fit in your backpack? Had a second primary weapon in hand when finding this glitch.

Drag one item over the other and combine the two items while keeping them on the floor .. results in both items remaining AND the new crafted item placed on the ground.

End Result in my case was endless full syringes, but it's possible this is a active glitch to create endless splints, arrows, improvised bows, ect.

Made this a private ticket for now, don't know if that's the correct way to do it but didn't want to make this a well known exploit.