It appears as though today's Destiny update (version 1.0.3) is causing some Exotic items to behave, as developer Bungie puts it, "differently."
Although Bungie has previously mentioned addressing underperforming Exotic weapons, they've yet to act on it yet. Any changes you might be noticing as a result of today's update are "unintential."
Responding to numerous post-update complaints regarding Exotic items, Bungie shared the following message with players: "If you're noticing Exotics behaving differently, the changes are unintentional. The Weekly Update will cover incoming Exotic tuning."
Bungie didn't get into any specifics, but the fine folks over at the Destiny subreddit are compiling a list of "hidden" Exotic weapon changes they've noticed so far. Included in their list, but not actually confirmed by Bungie, are nerfs to Thorn's DoT (damage over time) and base damage, The Last Word's damage, and Pocket Infinity's scope.
Bungie community manager David "DeeJ" Dague has said the developer is investigating these issues. As always, we'll update you if we learn anything more. In the meantime, you can check out the official patch notes for today's update here.
