A surefire double play turned into a throwing error charged to Parnell in what became a four-run seventh inning, and the Pirates completed their first sweep in Queens in 11 years with an 8-1 victory before an announced crowd of 40,250.
The Mets (63-55) again had their division lead trimmed to four games, pending the outcome of the Washington Nationals' late-afternoon game in San Francisco.
The Amazin's actually were swept by Pittsburgh in the six-game season series. The Mets finished 3-17 against the three seemingly playoff-bound NL Central teams -- Pittsburgh, Chicago and St. Louis.
Taking over for Matt Harvey with the score tied at 1 to begin the seventh, Parnell walked No. 8 batter Pedro Florimon. Parnell then coaxed a comebacker from pinch-hitter Michael Morse that seemed destined to result in a double play. However, shortstop Ruben Tejada appeared late covering second and Parnell's throw went into center field, placing runners on the corners with none out. Officially, the error was assigned to Parnell.
Parnell ultimately was charged with four runs (two earned) while recording two outs. Gregory Polanco delivered a tiebreaking RBI single against Parnell. A passed ball charged to Travis d'Arnaud gave Pittsburgh a two-run lead. Aramis Ramirez's RBI single chased Parnell. Eric O'Flaherty then allowed a baserunner inherited from Parnell to score on Pedro Alvarez's single as Pittsburgh took a 5-1 lead.
Parnell (1-3) also suffered Friday's loss. He surrendered singles to all three batters he faced in the 10th inning and was charged with two runs in the series opener.
Harvey's day: Matt Harvey now has 16 career starts in which he allowed one run or fewer but did not get a win. That is tied for the fourth-most in the majors since the start of the 2012 season, despite Harvey missing all of last year while recovering from Tommy John surgery. The only starting pitchers with more non-wins in appearances allowing one run or fewer over that span: Felix Hernandez (22), Zack Greinke (22) and Jeff Samardzija (20).
Harvey's scoreless streak ended at 16 innings when Alvarez launched a leadoff homer in the second inning to open the game's scoring. That was the only run Harvey surrendered in his 103-pitch no-decision.
Harvey twice allowed two-out triples -- to Starling Marte in the third and Polanco in the fifth -- but stranded both.
Harvey allowed one run on seven hits and one walk and struck out six in six innings. He departed with the score tied at 1 thanks to a second-inning solo homer from d'Arnaud against Jeff Locke.
What's next: The Mets travel to Baltimore late Sunday afternoon but do not face the Baltimore Orioles until Tuesday. Jacob deGrom (11-6, 2.03 ERA) opposes right-hander Kevin Gausman (2-4, 4.48) in the opener of the two-game series.