Papelbon's stuff was especially nasty during his prime years, which were 2006-08. In September 2007, when the Sox were clinging to first place over a surging Yankees team, Papelbon walked 0 batters in 11 innings. He had one bad outing, when the Sox had a 7-2 lead over the Yankees in the 8th. Okajima blew up (2 HR's, a walk, and a single), and Papelbon came on to put the fire out, and failed. But he was facing the varsity: Jeter, Abreu, and A-Rod, and sometimes those guys win. But he was otherwise lights out, and didn't allow a single run in 10 2/3 post-season innings, and was even more dominant in the 2008 playoffs (13K's, 2 BB's in 10 1/3 innings).
The first real chink in his armor came in the 2009 ALDS against the Angels. The Sox were overmatched that series, but were ahead 5-2 in the 8th of Game 3 in Fenway. Papelbon came on with 2 on and 2 out, and Juan Rivera found a gap in right field, scoring both runners. The inning would end with the pinch runner getting picked off first base, and the Sox still ahead 5-4, and would add an insurance run in the bottom half of the inning.
In the top of the 9th, sporting a 2 run lead, Papelbon threw 29 pitches. Four of them were an intentional walk to Torii Hunter. Of the 25 thrown in anger, all of them were fastballs. After a bases clearing single by Vlad Guerrero gave the Angels a 6-5 lead, Papelbon was done, never again to pitch in the postseason. Of the 29 strike pitches thrown in the 8th and 9th, the Angels made contact on 12, with only one swinging strike. It remains a mystery to this day why Papelbon could not mix in any of his secondary breaking pitches, a strength of his during his prime days.
The walks ticked up in 2010 and 2011. But he was actually superb again in the stretch run in 2011, but blew two very costly games to the Orioles in September, as not even he could avoid the curse of the BABIP gods that struck nearly every pitcher in the staff that month.