A simple potion that has saved the life of many an arcane caster.
A magic user drinks this potion when prepares spells for the day. Whenever he casts a spell he rolls a Saving Throw against Magic. If the save is successful then the spell is retained in the magic user’s memory and the effect of the potion ends. If he fails the saving throw then potion does not activate.
Note: The magic user has no control which spell, he must save against. It is whenever he casts a spell, whether it be a simple cantrip or a powerful ritual.