Problem: When you dip a stack of ammunition into poison then fire it and pick it up again, the ammunition no longer stacks.
Desired solution: Poisoned ammo should continue to stack for as long as it is poisoned.

And getting into theorycrafting a bit:
Hypothesis: Items currently track how much poison is left on them, or something similar.
Proposed solution: Make items be 'poisoned' or 'not poisoned', and assign a % chance for poisoning to go away completely.