Wade Boggs leaving for the Yankees was highly unfortunate
Even for his time, Wade Boggs was a bit of a unicorn as a hitter. He, along with Tony Gwynn, weren't mashers by any stretch. However, the guy just continuously put up numbers by putting the ball in play a ton. Many thought that his career would start and end in Boston as he became synonymous with Red Sox baseball in his 11 seasons there.
Unfortunately, when it was time for Boggs to re-up with the Red Sox, Boston's owner passed away and that contract offer was taken off the table by the new owners of the Red Sox. So, after debating his options, he decided to sign with the Yankees and Boston fans never really forgave him for it. It took Boston a long time to even retire his number despite him meeting all the known criteria. Never underestimate the Red Sox and their fans' ability to hold a grudge.
Luis Tiant spent the twilight of his career with the Yankees
This one is a bit less severe, but still worth mentioning. Luis Tiant spent seven years in the big leagues before he joined the Red Sox for the 1971 season. Over the next eight season, Tiant would finish in the top six of Cy Young voting three times and post a 3.36 ERA with 1075 strikeouts with the Red Sox. His 1972 season is the stuff of legend where he posted a 1.91 ERA despite not even being in the rotation for the first half of the season.
When the time came for him to decide if he was going to stick around, he instead decided to take a two year deal with the Yankees. In fairness to Tiant, he was in the twilight of his career and was just looking for one more decent payday before he retired. He also has stuck around the Red Sox franchise in his retirement. However, there are a number of fans that haven't forgotten that he jumped ship for the hated Yankees.
Johnny Damon signing with the Yankees may have been the biggest betrayal of all
Johnny Damon started his career with the Royals, but his time in Boston saw him personify the Red Sox "idiots" that turned Boston from bridesmaids into finally winning a World Series title again. In his four seasons with the Red Sox, he went to two All-Star Games and slashed .295/.362/.441 with 56 homers and 98 stolen bases. Many thought he was going to be in Boston for a long time.
It was then shocking that when Damon became a free agent, he decided to signed with the Yankees for four years and $52 million. Not only were Boston fans upset that the Red Sox just lost one of their best players, but he decided to go to the Yankees of all teams. Damon would play pretty well in New York for four seasons before bouncing around the last few years of his career.
The worst part of Damon's betrayal, though, was that he said he would never sign with the Yankees. He went on the record to say that he wasn't looking to get the most money possible if that meant he had to sign in New York. Well, that is exactly what he did and Red Sox fans will never let him forget it.