Doc Rivers has coached SIX teams that have blown 3-1 or 3-2 series leads:

2020 Round 2 vs Nuggets, 3-1 lead
2015 Round 2 vs Rockets, 3-1 lead
2012 East Finals vs Heat, 3-2 lead
2010 NBA Finals vs Lakers, 3-2 lead
2009 Round 2 vs Magic, 3-2 lead
2003 Round 1 vs Pistons, 3-1 lead