#7 - Acquiring the injury-prone Mark Mulder for a multi-time All-Star
It's hard to fault a team for "going for it" when they see their championship window. In 2005, the Cardinals' acquired a young star in Mark Mulder, who finished 2nd in Cy Young voting in 2001 and was an All-Star in 2003 and 2004.
Unfortuantely for St. Louis, Mulder had already peaked.
After going 16-8 with a 3.64 ERA in 2005, Mulder would struggle with injuries and terrible performance before his big league career would end in 2008. Mulder's career took a downturn that few could've predicted, but what hurts even more are the assets they gave up to get him.
Outside of giving up two useful players in the forms of Daric Barton and Kiko Calero, Dan Haren would emerge from a so-so young starter with the Cardinals into a three-time All-Star, two-time top 7 finisher in Cy Young voting player over his 13 big league seasons. The success for Haren came pretty quickly, as he was a very similar pitcher to Mulder in 2005 and earned his first All-Star appearance in 2007.