I bet the most common uncompleted triumph is the "Chest Hunter". I'm a day one Destiny player and I went to go check my moments of triumphs to see if I had already completed all of them. I was happy to see that everything was completed, except for the 4 missing golden chests out of the 20 from the triumph "Chest Hunter". Well, time to go hunting.
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Took me like 30 mins to get 7 chests with YouTube videos. It was pretty obvious to me which ones I was missing