Buffalo rookie Robert Mongrain gave the Washington Capitals a giant headache last night. The free agent out of Three Rivers, Quebec, who is not even listed in the Sabres' press guide, scored an unassisted goal with 3 minutes 44 seconds to play to shatter a 3-3 tie. After empty-net score, the Sabres were 5-3 winners and the Capitals were shaking their heads over a wasted effort in which they outshot the talented visitors, 31-23.

Within 2 minutes 22 seconds early in the second period, three pucks and one paper airplane sailed past Gary Inness into the Washington net. The pucks gave Buffalo a 3-0 lead and the airplane indicated the subsequent disinterest that suddenly reigned among the 11,216 Capital Centre fans, many attracted by a two-for-one admission bargain.

Buffalo entered the game with the best defensive record in the NHL, only 2.50 goals per game, and its goaltender, Bob Sauve, was the No. 2 individual with a 2.06 goals-against mark. Accordingly, there was little expectation of a Capital comeback.

It came just the same. In a span of 3 minutes 23 seconds, still shy of the midpoint of the second period, Mark Lofthouse, Mike Gartner and Rolf Edberg scored to pull Washington even.

Once tied the game degenerated into a series of minor penalties. The Capitals' newly constituted power play, with Rick Green at right wing and Guy Charron at the left point, extended its 0-for-21 streak to 0-for-27 and Buffalo did not add an extra score to an early such effort by Gil Perreault until it was empty-net time.

The fans had their opportunity to dream, however. When Buffalo's Larry Playfair dumped frequent adversary Robert Picard and was tabbed for interference, he chose to chase Picard rather than go to the penalty box. Referee Bruce Hood added the mandatory bench minor and Buffalo was scheduled to be two men short for two minutes.

Instead of glory, the Capitals covered themselves with embarrassment. After 1 minute 20 seconds of steady, unfulfilled pressure, the Sabres managed to ice the puck. The Capitals made a massive player change and, with the power play personnel unacquainted, Edberg came out to replace Rick Green, who didn't leave.

The Capitals were slapped with a minor penalty for too many men and shortly thereafter lined up for the faceoff. They now had only three skaters in front of Gary Inness and one of the trio, Bob Sirois, had to skate to the bench, waving three fingers in the air, before Coach Danny Belisle realized surplus had deteriorated into shortage.

It was still 3-3 when a Washington shot circles the boards and left the Buffalo zone. Green could not handle the puck and it came into the possession, ever so briefly, of the Capitals' Yvon Labre. Mongrain stripped it from him, skated to the faceoff spot in the left-wing circle and wristed a drive through Inness.

Further embarrassement acrued to Picard, victim of some fancy skating by Danny Gare, who went around him toward the Washington net. Labre snuffed the breakaway at the last moment, but in the process was whistled for tripping with only 1:21 remaining.

With the puck in the Sabres' end, Inness was yanked anyway for a fifth skater. Buffalo's Don Luce won the faceoff and Craig Ramsey soon was headed over the Washington blue line, with Picard backing cautiously toward the vacated net. Ramsey suddenly faked one way and went the other, netting the clincher while Picard leaned back on his heels.

Afterward, the Capitals could think back to so many lost opportunities, notably two shorthanded breakaways on which Bengt Gustafsson was foiled by Sauve. Picard had seven shots without a score and there were other solid attempts blocked by a makeshift defense that glued the Sabres together in the absence of injured stalwarts Jerry Korab and Jim Schoenfeld.

"We're that close all the time," Belisle said. "That's the discouraging part. We don't get any of those lucky goals, it seems. We've got to keep to our system until something breaks for us."

"That's probably the best game we've been involved in all season." said Buffalo Coach Scotty Bowman.

It is too bad the result prevented so many spectators from appreciating it.