I opened 4 hidden chests along my way through the plate puzzle rooms in Thunderhead on my engineer without noticing it was a daily. After getting the final treasure, I left the map and swapped to my ranger. Then I noticed the daily achievement at 80%/4 of 5 chests opened. Unable to do anything about it with my ranger, I swapped back to engineer. I had hoped the 15 minutes or so of Light of Deldrimor buff/special skill were still there, but it seems that leaving the map or swapping had removed it. Now I appear to be stuck with a daily that is 80% done, and absolutely no way of completing it.
I certainly had a part in getting myself into this situation, but the design of this plate/puzzle system and its relation to exploration seems likely to put others in the same mess. The only reason I was using my engineer for the treasure room puzzle was to get into the purple/graveling room to unlock its POI. Being annoyed at not getting a purple plate, I parked the engi in Noble's Folly and resumed playing on ranger. Then I realize that not only did I fail to get the POI I was after, but I was now also blocked from completing a daily.