As you probably know, the a full vial of Polyformic Acid from Stratholme has 3 charges and is unique. You were not intended to leave Stratholme with more than 3 charges; this exploit allows you to leave with 6.
Because of how the flask works and each stage of the flask having a different item ID, you can bypass the unique requirement and have 3 flasks (with a total charge count of 6) by following these steps:
- Kill Professor Slate in normal Stratholme (5th boss)
- Loot the Empty Vial of Polyformic Acid
- Fill the vial by right-clicking the vial on the table next to the boss
- Use the vial in your bags, transforming you into Professor Slate
- Right-click your now-empty vial to fill it up, giving you a Half-Full Vial
- Repeat steps 4 & 5, this will yield a Nearly-Empty Vial (Charge count: 1)
- Reset the instance and repeat steps 1-5, this wield yield a Half-Full Vial in addition to your Nearly-Empty Vial (Charge count: 3)
- Reset the instance and repeat steps 1-3, this wield yield a Nearly-Full Vial in addition to the previous two (Charge count: 6)
If you followed the steps correctly, you will now have 3 different vials (Nearly-Empty, Half-Full, and Nearly-Full) in your bags. This will reduce the amount of times you have to re-visit Stratholme for the flask by 50% as you can now leave with twice as many charges.
Proof:
Have fun!