May 25, 2002: In the Eastern Conference Finals, the Nets lead the Celtics by 21 points after 3 quarters. The Celtics, led by Paul Pierce (19 of his 28 points came in the 4th) outscore the Nets 41-16 in the last quarter to win 94-90.
June 12, 2008: In the Finals, the Lakers led by as much as 24 points (and 20 points at halftime). Paul Pierce led all scorers with 20 points and hit the clinching free throws to finish comeback (allowing the Celtics to take a 3-1 lead on the way to a Championship).
May 12, 2009: In the Conference Semifinals, the Magic led by 14 points late in the game and were still up 10 points with 5:39 left in the game. The Celtics storm back to win the game (Pierce made a critical layup and played well on defense in another great all-round game by him).
See a common thread here?
I don't mean to take anything away from the other participants in any of those games. Antoine Walker helped in 2002. Eddie House was huge in the Finals comeback. Big Baby and Marbury led the way last night. (and team defense is often overlooked in all these types of wins) But Paul Pierce just has a knack for being in these types of games.
To be fair, you could question why his teams find themselves in a hole. To come back from a large deficit you first have to be down by a lot. However, I'll submit that everyone has bad games from time to time. Some chose to fight back. Some even succeed. Few seem to succeed as often as Paul Pierce.