It happens, but rarely. So far, I've gotten a special (77 wizard), a primary (ascendant energy), and a class (AOS#Cryptid). Two of them, coincidentally, came from the the same room. It's just luck.
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I was doing the nightfall last week (The Nexus) and after my buddies and I beat the boss, I ran around collecting my drops. I saw what looked like a blue (that boss room's coloring is wonky as shit) and it turned out to be a purple. Then, I took two steps to the left to continue my clean up just to pick up another purple! RNGesus loved me that day because I got two more purples as a reward for the weekly heroic level 28 right after I finished the nightfall. The only consequence to that extraordinary luck is that I didn't get a single piece of raid gear last week :(
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I got two legendaries from the dregs in the first room of the Devil's lair. It wasn't even the weekly, just a tiger...