If a player mixes ingredients in a retort or a flask and the ingredients share both positive and negative effects, the potion fails and the player learns nothing. Use of an alembic does not help in this situation because the player does not know what effects to filter out.
If, for some reason, an alchemist wants to make a low-potency potion, he or she may choose for the effect to be lessened by d6s, diminishing the potency by the result out of ten (ex: roll a 4, the potency is reduced by 4/10). The number of d6s must be chosen before rolling.