"Just overall, way too passive" in the latter stages of the third period.
Leading by 4-1, the Flyers allowed three goals in the last 8 minutes, 40 seconds of regulation to force overtime. The equalizer was scored by Alex Ovechkin with 47.9 seconds left, set up by goalie Steve Mason's turnover on a clearing attempt.
The Flyers appeared on their way to avenging last month's 7-0 loss before the Caps' late flurry, one in which they scored three goals in 7:52.
"I don't think we played really bad at the end; it was just the right bounces for them," said winger Michael Raffl after the first three-point game of his young career.
Sean Couturier and Jake Voracek scored 1:14 apart early in the third period to give the Flyers a 4-1 lead.
In the shootout, Couturier and Matt Read failed to score, while Claude Giroux had the Flyers' lone tally. They are 1-3 in shootouts, while Washington is 8-3.
Eric Fehr scored Washington's first shootout goal as his shot bounced off Mason's shoulder and trickled over the goal line.
"I got a good chunk of it, and I thought it was in the corner," Mason said.
Couturier gave the Flyers a 3-1 lead with 17:42 left, scoring from a nearly impossible angle from the goal line near the left boards, caroming the puck off rookie goalie Philipp Grubauer's left pad. Seventy-four seconds later, Voracek redirected Raffl's shot.
The Capitals got to within 4-2 on Mike Green's goal with 8:40 to go, and Dmitry Orlov's point drive beat Mason high to the short side, cutting the deficit to 4-3 with 3:31 remaining in regulation. Defenseman Nick Grossmann inadvertently screened Mason.
Mason made a key kick save on Troy Brouwer with about 1:35 left. But a short time later, Mason's turnover led to Ovechkin's tying goal from the high slot.
"That was a misplay," Mason said of his failed clearing attempt. "I didn't get much on it. A bad mistake."
In the first two periods, the Flyers threw up a defensive blanket in front of Mason, allowing a total of 14 shots. But the Caps had 16 shots in the third period.
The Caps had "too much perimeter stuff" until the third period, Green said. "Once we started peppering him with shots, one was bound to go in."
The Flyers get another crack at the Caps on Tuesday at the Wells Fargo Center.
"I think slowly our four lines are being built to play well together," said Giroux, who tied the game late in the first period. "But we have a lot of work to do."
Ovechkin, who leads the NHL with 27 goals, was injured and did not play in the Capitals' 7-0 win over the Flyers at the Wells Fargo Center on Nov. 1. The teams combined for 31 penalties, five fights, and 164 penalty minutes in that mismatch.
Sunday's rematch was much more tame: two fights and a total of 13 penalties for 38 minutes.