"This building will stand forever."Harsher in Hindsight is a serious event, plotline, or comment where a later event (in the story or real life) comes up and only serves to amplify the already bitter-tasting effect. It can lead to episodes or issues being pulled or held from re-running if it's perceived as Too Soon. This leads to a rather heart-wrenching moment and makes some people feel uneasy about it when they view the event in the original work. Compare "Funny Aneurysm" Moment, Too Soon, Life Imitates Art, Cerebus Retcon (elements that were originally comedic are later deconstructed and played as tragic). Note that if the event in question is something inevitable, such as people dying, that's not necessarily this trope. Everyone dies eventually, as far as we know. It would be this trope if the death had some link to how they appeared on screen. Contrast Hilarious in Hindsight, Heartwarming In Hindsight. CONTAINS SPOILERS.
Keep in mind, this trope is for stuff that was already dramatic/serious to begin with, but is now worse due to history. For stuff that started out funny and now isn't because of history, the correct trope is "Funny Aneurysm" Moment.