Default raid frames will normally hide long duration raid buffs such as Mark of the Wild, Fortitude, Divine Spirit, etc when you get into combat. This works for every rank of these spells except for the newest ones in Wrath.
This makes the raid frames very cluttered in combat and as a result much harder to track important short duration buffs such as HoTs and Shields. It’s a very minor UI bug but is very debilitating in raid environments where keeping relevant information clear and visible is important.