2010 Celtics (Game 7 Finals)
1973 Celtics (Game 7 ECF)
1987 Celtics (Game 4 Finals)
2011 Celtics (Trading Kendrick Perkins)
2013 Bruins (Game 6)
Oh sorry, you said last 20 years.
1973 and the lost FT rebound by McHale at the end of Game 4 in '87 were too painful for me to leave out.