The “Erfurter Latrinensturz” is a great candidate:

The Erfurt latrine disaster occurred on 26 July 1184, when Henry VI, King of Germany (later Holy Roman Emperor), held a Hoftag (informal assembly) in the Petersberg Citadel in Erfurt. On the morning of 26 July, the combined weight of the assembled nobles caused the wooden second story floor of the building to collapse and most of them fell through into the latrine cesspit below the ground floor, where about 60 of them drowned in liquid excrement. This event is called the Erfurter Latrinensturz (lit. ‘Erfurt latrine fall’) in several German sources.