The Hidden Forest

I've seen this a couple times, at random, but generally just brushed it off as a random bug people knew about, but it never occurred enough to worry about. However, tonight it ended up wiping us, so I'm gonna post and see what happens.

We were pulling Ikatiar in NToV, and I had a pre-heal landing on our main tank, Tinimus. When the heal landed, he was at about 40% hp, and I got a red text bar saying "You tried to cast a spell on this target, but they are protected", much like you would get for trying to cast damaging spells on other players. This isn't the first time I've seen this happen, but it's never been a huge issue until now.

Any idea what could be causing this?

Divine Aura the damage immune spell pallys clerics shadowknights and bards get

This heal was casting during the pull, I wasn't DA/DB'd, etc. Our tank is a Warrior, and was taking damage, or else we wouldn't have wiped, so it wasn't Bestow DA, either.