Curse of Bobby Layne
In 1958, the Detroit Lions traded quarterback Bobby Layne to the Pittsburgh Steelers. Following the trade, Layne reportedly stated that the Lions would not win a championship for 50 years. In those 50 years that followed, they went 1-10 in the playoffs, and are still one of only two NFL franchises that have been in the league since at least 1970 and have never made a Super Bowl appearance (the other is the Cleveland Browns, sorry guys).
How did the Lions do in the last year of the supposed curse, 2008? Well, they went 0-16, but they did draft QB Matthew Stafford the next season.