Criticize and/or add your own!
Cautious casting: The magus is treated as having the Cautious Caster Virtue while casting the spell.
Reckless casting: The caster can choose to boost the spell one magnitude in puissance, with the Storyguide determining the exact effect as per Wizard's Boost (this effect can be stacked with that of a Wizard's Boost). If he does, however, he also rolls twice the usual number of botch dice, with a minimum of three botch dice, after all other modifiers to botch dice have been applied.
Parsimonious casting: The ability can only be taken once for a given spell, which must be a Ritual and can then be cast using one less pawn than normal.
Teaching insight: The magus is treated as having the Good Teacher Virtue when teaching or writing about the mastery ability. When teaching the spell itself, only half its level is counted for the purpose of determining how many spells the student can learn during a season.
Special Circumstances: The spell benefits from a casting bonus of +3 under a special set of circumstances. These must be no more common than those of the "Special Circumstances" Virtue in relation to the spell (e.g. one cannot designate "when touching the target" as a Special Circumstance for a R:Touch spell). The bonus provided by the Mastery Ability is not cumulative with that provided by the Virtue. Like in the case of the Virtue, this ability can be chosen multiple times, each time selecting a different set of circumstances; if multiple sets of circumstances apply to the same casting, the caster benefits from a single +3 bonus.
Theoretical underpinnings: When casting a similar spell spontaneously, the caster adds his Mastery score to the Casting Total. A spontaneous spell can benefit from only one such bonus at a time. (Note: this rings all my alarm bells about potential abuse, but I haven't found a concrete case).
Communal casting: When casting this spell in a Wizard's communion, the caster subtracts one botch die for each participant, to a maximum of his Mastery score (eliminating the corresponding die added via the Communion).
Tailored casting: When choosing this ability, the magus selects a number of valid targets for the spell equal to his Mastery score, and can subsequently add one more target whenever his Mastery score increases by one. The spell is treated as specifically tailored to any of these targets for the purpose of determining Warping.
Experimentation. The magus explores uncharted ground about the possibilities of the spell. When selecting this ability, roll on the Extraordinary Results chart, treating the number of times this mastery ability has been selected as the Risk modifier (the ability can be selected a maximum of three times).
Disaster: the magus score in the Mastery ability drops by 1 (the "Experimentation" ability is lost as a result in the process).
No extraordinary effects: the magus must select another mastery ability.
No benefit: the Experimentation ability does nothing (but still "wastes" the special ability tied to this level of Mastery)
Side effect or Modified Effect: The magus learns how to cast a variant of the spell, determined according to the Side effect/Modified effect subtables (it is effectively the same spell, so the Mastery score and abilities apply to both variants). The magus retains the ability to cast the unmodified spell.
Discovery or Special/Story Event: as per the standard Experimentation results.