So during the Sindragosa encounter in ICC the most enfuriating thing for a healer is to sit there and not heal while people die because you have too many stacks of unchained magic.
As a resto druid there is a way to cheat this somewhat. When you get targetted for Unchained Magic, cast regrowth- just before it finishes casting, cast rejuvenation. If you did it correctly the two spells should land at approximately the same time and you will notice you only get 1 stack of Unchained Magic. This lets you cast 10-12 spells and only get 5-6 stacks of unchained magic.
I think the server doesnt register them for unchained magic if they land so closely together. I did a quick search and couldnt find anything about this. Small exploit, but usefull. May work for other casters with similar spells (one with a casting time and one instant)
Thanks and good luck