Hi there,
Shadow priests and Resto druids will be able to keep 100% mana forever due to the way this trinket works (unintended, else it'd be way OP and the biggest flaw Blizz ever made)
Ephemeral Snowflake - Item - World of Warcraft
This trinket does not only procc from your direct heals, it also proccs from your HoT ticks (Shadowpriests' Vampiric Embrace), meaning 11 mana back per tick = infinite mana back.