Hey! Thanks for playing. This is one major benefit to using Inform 7 - the parser is super helpful to the player right out of the box, and even better with a couple extensions added. You should be able to work through the potions puzzle with the HINT system should you ever wish to try again. :)
Also, yes you did find a bug. I added killing the guard last minute (when I played after a break and realized that's what I would try!) and apparently it breaks some things. Thanks for finding that.