Since last Season I have found one event and several cursed chests around the landscape that cannot be successfully cleared.
I first noticed this on the Bounty mission, A Plague of Burrowers. There is no timer on this event other than the one that affords you an additional chest, so you could take all day to clear the map and kill all the Burrowers. After going around the map several times, I found no additional enemies to kill, and I had to quit the game in order to reset it, without successfully completing the event.
Sometimes, with random cursed chests, there may be a monster hiding behind a bush somewhere, but not in these cases. Even in a captive area such as the Pandemonium fortress, where an entire map area can be easily cleared, the chest remains cursed, there remain no additional enemies to kill, and the chest will time out and fail.
Just a dev FYI.