The result might prove much more costly for the Senators, who continue to slide further away from a playoff spot.
Ottawa tries to bounce back from a stunning defeat when it hosts Colorado on Sunday.
The Senators (28-25-13) have dropped six of eight and are six points behind Philadelphia for the second wild-card spot in the Eastern Conference. They squandered a three-goal lead Saturday at Montreal, giving up three unanswered goals in the final 3:22 of the third en route to a 5-4 overtime loss.
David Desharnais forced the extra period with a power-play goal with one second left.
"I don't really know what to say," said Marc Methot, who ranks second among Ottawa defenseman on the season with a plus-1 rating but is minus-5 over his last six games. "It felt like we were in control, even in the third period. We weren't playing on our heels, at least for a good amount of it."
Francis Bouillon beat Robin Lehner for the winner at 1:26 of overtime, marking his only goal in 40 contests this season. But Lehner -- who made 43 saves while starting in place of Craig Anderson, who suffered a shoulder injury in Monday's loss to Nashville -- was irate afterward. He and teammates protested that he froze the puck before it came free and found Bouillon's stick.
Captain Jason Spezza said officials told him the play wasn't whistled dead because the building was too loud.
"I have no idea what that means," coach Paul MacLean said. "That's the explanation we got. I'm confused."
Sitting in second place in the Central Division, the Avalanche (43-19-5) might be more poised to bounce back from their own deflating defeat. All of Anaheim's goals in a 6-4 home loss to the Ducks on Friday came in the second period. It marked the most goals allowed in a period by the franchise since the Quebec Nordiques surrendered seven in the second period of a 7-5 loss to Pittsburgh on Feb. 27, 1995.
Semyon Varlamov allowed five goals during an 8:45 stretch before being pulled in favor of Jean-Sebastien Giguere. Varlamov had posted a 2.16 goals-against average and .939 save percentage over his previous seven starts.
"We couldn't stop the bleeding," coach Patrick Roy said. "Look at the fifth goal. It hit the backboards and goes in."
Colorado scored three times in the second, including two goals from Jamie McGinn after he had scored just once over his previous 12 games. Gabriel Landeskog also recorded a goal and an assist, ending a five-game stretch without a point that matched the second-longest of his career.
Paul Stastny was sidelined for a third straight game (back) and is day-to-day.
The Avalanche's 18 road points (9-3-0) since Jan. 11 are the league's second-most during that span.
Colorado won a 4-3 meeting in overtime at home on Jan. 8 for its sixth straight victory in the series. Tyson Barrie scored 33 seconds into overtime, while Stastny recorded a goal and two assists. Landeskog was credited with a career-high three assists and also had a goal and an assist in a 7-1 victory on Oct. 13, 2011.