19. "The Invitations," Seinfeld
Air Date: 5/16/96
Most sitcoms are interested in preserving the status quo. Plenty of dramedies like Friends or The Office move to progress their characters' relationships, but for straight-up comedies that thrive mostly on their formula like Seinfeld, developments such as marriage are simply out of the question.
The problem is, George Costanza (Jason Alexander) is just too much of a non-confrontational weasel to end the engagement he never really wanted to Susan Ross (Heidi Swedberg). His freedom from the impending nuptials comes by way of one of the most controversial twists of fate in sitcom history, when George's unrelenting, character-defining cheapness inadvertently leads to Susan's death, thanks to a remarkably inexpensive batch of wedding envelopes that turned out to be toxic. (She died from licking them.)
Seinfeld and the gang were always an amoral, apathetic bunch, but America wasn't quite ready for their understated, barely-there reaction to Susan's sudden demise, but years later the general consensus is that it was a darkly genius move on Larry David's part, and really, the only way the situation could have resolved itself, thematically, anyway. We could probably come up with fifty different ways we'd have envisioned Susan dying before we got to "toxic envelopes."