Mouldy Rye Butter
Reroll when the die does not land flat, or when it lands on the floor. All d20 rolls must be done on the mat! If it is not on the mat, you may be asked to reroll.
There is little regard for calling interference in this campaign. I don’t care if it hit a book or a Mountain Dew or a dancing homunculus. As far as I’m concerned, it only makes the result more random.