Hindsight is 20/20. If most people had a crystal ball and could see how things turn out, they might alter their decisions. This holds particularly true in sports.
There is no way to know how a player will perform or if a team should hold out or cut their losses or go after a certain player. Sure, there are stats and scouts that can inform a team on how to make a good decision, but that doesn't mean those projected numbers will translate into success.
When the Yankees traded for Randy Johnson in 2005, he was a World Champion, on his way to Cooperstown, Cy Young Award winning, dominating, left-handed pitcher. He was going to be the ace of the staff and provide stability that had been lacking since the departure of Pettitte and Clemens after the 2003 season.
That trade didn't work out so well. Johnson couldn't adapt to playing in New York, and as a result, his performance suffered. There was no way of knowing the trade would be a bust, and everyone hoped that Johnson's experience and proven success would show up at some point. Instead, before his contract was up, he agreed to a trade to go back to Arizona.
Then there are trades that really work out. Take, for example, the Red Sox in the 2006 offseason. They made a deal with the Marlins for Josh Beckett, and the Marlins would only agree if the Red Sox took third baseman Mike Lowell off of their payroll. Boston agreed, and two years later, Beckett and Lowell won a World Series as Red Sox.
Beckett has been stellar for the Red Sox, but in many ways, Lowell has been more valuable. He is on the field for every game, while Beckett only pitches every five days or so. Lowell is a Gold Glove winning third baseman who has been clutch on both sides of the ball, and the Marlins wanted him off of their hands.
There are plenty of other examples of good and bad trades. Manny Ramirez going to the Dodgers and Jason Bay going to the Red Sox would be viewed as a good trade by most people.
The Giants trading Joe Nathan, Boof Bonser, and Francisco Liriano to the Twins for A.J. Pierzynski would be a bad trade for the Giants that worked out amazingly for the Twins, especially where Joe Nathan is concerned. He has become one of the most dominating closers in the A.L.
Both the Rays and Phillies have benefited from good trades. The Phillies acquired Joe Blanton from Oakland in July, and he pitched a solid Game Four on Sunday night. The Rays traded for Matt Garza and Jason Bartlett in the 2007 offseason and Garza ended up as the MVP of the ALCS, going 2-0 and taking down the Red Sox.
Bartlett has provided stability to the infield and more speed in the lineup. The Rays also were involved in a trade with the Mets a few years back, where the Mets were after Victor Zambrano. In return, the Rays got a young lefty named Scott Kazmir, who was slated to be a very talented pitcher.
Kazmir was the winning pitcher in the 2008 All-Star Game and was the starting pitcher in Game One and Game Five of the World Series. I'm guessing the Mets are kicking themselves on that one, but it comes down to one thing...