That day in 1992 when Sid Bream -- one of the slowest men in baseball and a former Pirate -- crossed home plate for Atlanta to send them to the World Series over the Pirates. Came back from 3-1 to tie the series and had the lead in the bottom of the ninth, but things just went to hell...
Not only was it a bad day (was a freshman in college and quarter-beer day at one of the bars that didn't card was the only thing that got me through that night), but it directly led into their long-ass streak of losing seasons where they didn't sniff the playoffs again until 2013.
Can't think of any one single day that sucked that badly.