(Video from HockeyVideoHD)
The Sabres' power play came after a bad call by the officials, who whistled Montreal's P.K. Subban for high-sticking. As you can clearly see in the video above, his stick never actually made contact with the Sabres player.
If Subban had made contact there, with his stick or his arm, then a penalty would have been warranted. But the fact remains that Subban didn't touch him, so this was just a bad call.
Montreal still got a point in the contest and remained in first place in their division by staying ahead of the Boston Bruins, but losing like this has to sting.