The Massacre of Tyrell's

The Massacre of the Tyrell's was a series of events initiated in King's Landing by Lord Rickard Karstark during the near end of the Interlude Period, around 346AB.
This event resulted in the massacre of around 30 Tyrell Military Forces, as-well-as Royal Family members, and led to the 3 hour occupation of the Red Keep under House Karstark, where Lord Rickard Karstark was briefly coronated Lord of the Seven Kingdoms and King of the Andals and First men.
Multiple factions aided both sides:
Tyrell Faction: 
House Tyrell
House Dayne
House Bracken (briefly)
House Drumm
House Targaryen
Karstark Faction
House Karstark
House Hersy
Mercenaries
The Toxic Era
Result: Karstark victory, 30 Tyrell's dead in King's Landing