LogFAQs > #975004810

This may be already known, but I haven't seen it mentioned in any list of glitches that I've seen.

Select a potion so that you have the option to use or discard it. Then save and quit. The next time you load a file, you will still have the option to either use or discard the potion I that slot - whether you actually have one or not.

The best benefit of this is to have one save file with an Entropic Brew, and switch over to a different save file, and drink the phantom potion to immediately get three free potions. You could also do the glitch at a shop that's selling an Entropic Brew or when you get it as a combat reward; get the potion, perform the glitch, and drink it when you reload the game.