Hello.
- Mental Agility: Discipline Priest talent that reduces the mana cost of Instant Cast Spells by 2%-10% (with 1-5 points]
- Surge of Light: Priest Wrist Rune that reads "Critical spellcasts cause your next Smite or Flash Heal cast within 15 sec to be instant cast.
From reading these two abilities, one would expect that, upon casting a critical hit spell (proccing Surge of Light), Flash Heal and Smite would have their mana costs reduced by 10%.
However, upon testing, I found that neither does the tooltip reflect this interaction, nor does it actually take effect (upon casting Smite and Flash Heal, both consumed the original mana cost, as if they were not instant cast).
I opened a ticket asking about this interaction, to which the GM replied it was not an intended interaction and directed me to this forum to post it. (technically, he directed me to the US forum, but yeah)