Some event happens in a work, and then a later event, in either another work or Real Life (often through Reality Subtext or Life Imitates Art), comes along and makes that first event give you warm and fuzzy feelings. Even if the event was already meant to be heartwarming, this makes it even more so. Heck, some works that Taste Like Diabetes at first have something come along and make them truly heartwarming. Compare Heartwarming Moments (which are heartwarming upon first watching), Hilarious in Hindsight, Fridge Brilliance.