Red passing, though it was not surprising, was still a huge loss for the franchise.
Losing to the Nets in the 2002 ECF. After the miraculous game 2 comeback, I really thought we'd get to see a Celtics Lakers finals, but somehow the Nets snuck in the wins. Didn't help that I've always strongly disliked Kenyon Martin.
Of course, in the end it probably just saved us from being swept by the Lakers. That would have been worse.
I thought losing to the Magic last year was also pretty bad. Seeing this team grit it out for so long, you had to wonder if there was just enough in the tank to go all the way, maybe even without KG. But ultimately it was just too much for any amount of heroics.