When You Get an Exotic Out of a Legendary Engram
Legendary engrams are nice, but exotics are better. The Taken King has overhauled how exotics work, leading to the peculiar situation in which uncommons and rares can be as good as or better than legendries and you can still only equip a single exotic weapon and armour piece at any given time. Still, there are few things in Destiny better than your purple engram transforming into yellow.