Kamiel Cornelissen set a very dubious record at Pro Tour San Diego. After putting up a remarkable 7-0 record on Day 1, he dropped all seven of his Day 2 matches. 6-0 followed by 0-6 has been done a couple of times at Worlds, most notoriously by yours truly at Worlds 1999 Day 2, but my record was tied in Toronto this past season by Joost Vollebregt.
There's a new record holder in town now, though. Cornelissen did us both one better, dropping the last round to Zvi Mowshowitz to complete his downward spiral.