Leclaire made 29 saves and Chris Neil scored Ottawa's third unanswered goal 6:15 into the third period as the Senators ended Montreal's four-game winning streak with a 4-2 victory Monday night.
"I think it went off Milan [Michalek] up in the air and then when I came in, I whacked it in," Neil said. "I had to wait for it to come down, too."
A lengthy video review supported referee Bill McCreary's ruling that Neil had not made contact with the puck above the level of the crossbar.
"I didn't know what was going on," said Halak, who was selected the NHL's First Star of the Week earlier in the day. "I knew I stopped the first shot and then I didn't know where the puck was, and unfortunately for us it ended up in the net -- too bad it wasn't a high stick."
Peter Regin had a goal and an assist and Ryan Shannon and Chris Campoli also scored for Ottawa, which won its second in a row as Leclaire came up with a strong outing in his third straight start since returning from a broken cheekbone.
"[Leclaire] played great," Fisher said. "He made a number of big stops and had a great game for us and kept us in it. If it wasn't for him, especially after that first period, we would have been down by more than a few."
Halak, who made his fifth straight start, made 24 saves after facing an average of 46.5 shots during Montreal's winning streak.
Shannon drew Ottawa even at 2 with a power-play goal 15:33 into the second. Regin picked up his second point of the game with an assist on the play after he got the Senators back into the game with his fourth goal at 12:48 of the first.
"We played a great 10 minutes, the first 10 minutes of the game, and then we stopped skating and started taking bad penalties," Halak said. "You can't win the game sitting in the penalty box."
Ottawa did not take a penalty, while Montreal was called for five minors.
"I think the key tonight, we played pretty disciplined," Leclaire said.
Cammalleri got the milestone 20,000th goal when he opened the scoring at 4:59, finishing off a fine passing play initiated by Andrei Markov and Andrei Kostitsyn with a shot from the slot into a wide open net for his team-leading 19th goal.
"I'm the kind of guy who likes the romantic aspect of hockey," said Cammalleri, who signed a five-year contract with the Canadiens as a free agent on July 1. "I'm glad I could be a part of this organization and this is extremely special."
The 20,000-goal total includes all of Montreal's regular-season goals since joining the NHL as a founding member in 1917, as well as the Canadiens' regular-season goals from their first eight seasons in the National Hockey Association, the NHL's predecessor.
Pouliot made it a two-goal lead with his third goal at 9:50, his first with Montreal since he was acquired in a trade with Minnesota.
Regin drew the Senators within 2-1 with his fourth goal at 12:48, moments after the huge contingent of Canadiens fans began chanting "Go Habs, Go!"
Leclaire kept Ottawa within a goal with several big stops, particularly late in the opening period.
"We scored that goal and that kind of gave us a little momentum and we had a hockey game," Leclaire said. "I felt pretty good today. Since I came back, I've been feeling good in front of my net. I had a few bounces go my way, too, a post and a couple of bounces that kind of rolled but it's one of those nights where things were going well so we'll take it."
"You've got to give credit to Leclaire, for sure," Pouliot said. "We had a lot of good chances and he came up big for them."
Shannon tied it 15:33 into the second when he scored off a faceoff win by Regin in the Canadiens' zone, five seconds after Jaroslav Spacek was sent off for tripping.
The Senators had a potential go-ahead goal by Fisher disallowed with 24.9 seconds left in the second. Referee Kerry Fraser immediately waved off the goal, ruling that Neil was in the crease and made incidental contact with Halak.
Campoli scored into an empty net with 46 seconds remaining.
Hall of Famer Newsy Lalonde scored the first goal in Canadiens history in the team's NHA debut on Jan. 19, 1910, 5:00 into a 9-4 loss to Renfrew. The Canadiens, who have won a record 24 Stanley Cups, celebrated the 100th anniversary of their foundation on Dec. 4. ... Guy Lafleur scored the Canadiens' 10,000th goal on Feb. 7. 1973, Montreal's third goal in a 5-2 win over Pittsburgh. ... Canadiens RW Georges Laraque was a healthy scratch. ... Montreal G Carey Price briefly went to the dressing room before returning after he was hit on the ear by a puck while sitting on the bench early in the first.