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!"
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.