The risk of any trade is that the player leaving your team will wind up having the more productive future than the player that you received.
Major League Baseball is full of these stories.
Remember when the San Francisco Giants traded Francisco Liriano, Joe Nathan and Boof Bonser for A.J. Pierzynski? I'm sure the Giants regretted moving those three.
If you are an Oakland A's fan, the first two names that probably come to mind as players that got away are Andre Ethier (Milton Bradley came to Oakland in return) and Carlos Gonzalez (along with Huston Street and Greg Smith for Matt Holliday).
Although Bradley did help the A's reach the American League Championship Series in 2006, you could argue that the A's would have benefited more from having Ethier in their outfield the following four seasons instead.
The Pittsburgh Pirates are among a handful of teams that gave up on Jose Bautista before he became the most dangerous hitter on the planet.
The Boston Red Sox probably did not realize the type of players Hanley Ramirez and Anibal Sanchez would become when they dealt them to the Florida Marlins for Josh Beckett and Mike Lowell. Although, you could argue that trade did work out for Boston as well.
Of course, it's not just prospects that get dealt for a big name. Sometimes the regret is for the team that trades away their star because they feel they won't be able to afford re-signing them after the season.
Here are a few players that could wind up on the move this season that their former team will wind up regretting letting them go.