I actually recreated this. Your MP must be greater than the potion your using so that potion disappears. I used a duelers essence.
1. get into a fight
2. Use more than 2000k MP.
3. Use MP heal while still in battle
4. potion disappears
5. You get hit on next turn
6. Look at MP..same MP as before you used the pot.
Getting hit right after using MP Heal takes away the MP you just gave yourself using potion. This only seems to happen when using the entire potion and it disappears.
All it needs to do is use the potion flask entirely during battle, it doesn't matter if it uses 30, 333 or 2000 at once, so long as the item poofs away. Anyway... it was missed a while back by myself and Jon. I've pointed it out a few times, along with a certain other one.
As an example of another, Use an empty potion flask (no potion equipped) to heal your non-full health outside of battle. All your mods are reset back to base and you need to equip/dequip something to return them to normal. If your HP before equiping/dequiping is now above your regular maximum, try to heal with the non-existent potion again, and your empty flask now has a value in it!
I even made a screenshot of this one just for j00!:
http://img150.imageshack.us/img150/6527/flaskdetailedstatsmodsrt5.png
