The 2010 Celtics before their playoff run.
That team stopped trying mid-season, and it really p---ed me off. I still remember one game in particular where I had enough. They were playing the Grizzlies, who were pretty meh that year, in Boston. The Celtics were dominated from the beginning by the likes of Rudy Gay, OJ Mayo, Marcus Williams, and Sam Young. Not only that, but it was the most egregious lack of effort I had ever seen as a Celtics fan. I can take being blown out, but I can't take that my team doesn't even care to fight back.
Throughout the entire game, I threw every bad word/phrase at the TV that came to my head at the Celtics. I can't remember the last time I told something to go F itself more than I did at the Celtics that night. For the first and only time ever as a fan, I gave up on them.
Sure they got their mojo back when they needed it, but man that team infuriated me.