1) Any accident or act of terrorism/vandalism that results in the destruction of perfectly good beer.
2) A mishap in the homebrewing of beer that results in a ruined batch.
3) Any mass-produced crap-beer like Coors, Bud, Miller, Heineken, Guinness, etc.
Dude, sorry you dropped your twelver. That was a total beertastrophe.
A beer related event considered catastrophic to serious beer drinkers.
Karl approached the bar and requested his favorite, high quality draft beer. The bartender pulled the tap and discovered the keg was empty. The bartender explained they were out of his beer and offered him a Bud Light. Karl thought, "what a beertastrophe!"
Paul was out with friends at their local pub. They were sitting at a table. The smoking hot waitress had just delivered a fresh round of pints for everyone. As Paul stood up to leave the table for the restroom, he accidentally bumped the table, causing six freshly poured pints to spill over. Brian belted out "party foul", but he was quickly corrected when Jim said, "dude, that's a beertastrophe!"