Sometimes, offseason acquisitions work out. Sometimes they don't. Sometimes, they are spectacular failures.
Brad Penny came to Boston for the 2009 season. He was one of four low-risk, high-reward players that the Red Sox signed that were surprisingly all lauded. However, Penny and fellow starter John Smoltz were massive disappointments. Rocco Baldelli was neither good, nor bad, and only Takashi Saito really improved the club markedly.
Penny had a bad year for the Dodgers in 2008 and was hurt a lot, but the two seasons before that he won 16 games each. He was brought in on a cheap deal and was expected to fill in the fourth starter spot behind Josh Beckett, Jon Lester, and Daisuke Matsuzaka. Unfortunately, other than a few starts here and there (particularly against the Yankees), Penny was pretty awful. In 24 starts, he had an ERA of 5.61 with a 1.534 WHIP. He allowed far too much contact and gave up too many home runs. His record was 7-8, but that was more due to the run support he received than effectiveness on the mound.
Penny was released by Boston in late August and caught on with the San Francisco Giants. Of course, he suddenly starting pitching well after that.