The Erfurt latrine disaster (German: Erfurter Latrinensturz, lit. 'Erfurt latrine fall') occurred on 26 July 1184 in the German city of Erfurt. Henry VI, then the King of Germany, was conducting a Hoftag ("court day") with local nobility on the second floor of a building. The combined weight of the assembled attendees caused the floor of the building to collapse through the ground floor and into the latrine cesspit below. Sources say that approximately sixty attendees died, some of whom drowned in human waste after falling into the cesspit.
Outhouses actually aren't all that bad if you maintain them properly, which involves tossing wood ash down on a regular basis to keep a high pH. I mean, it's not a toilet with running water, but I wouldn't rank it anywhere near the top of the worst things that I've smelled.