For a while, the NHL-leading Bruins looked like they would prevail, holding a 2-1 lead going into the third period. A victory would have given Boston a league-leading 40 wins (in only 55 contests) and its 20th on home ice.
But it didn't take long for the Sharks—now 10-1-1 against teams from the Eastern Conference—to rally in the final frame.
Patrick Marleau, Milan Michalek, and ex-Bruin Joe Thornton scored in the first 9:48 as San Jose stunned the Bruins for a 4-2 advantage by the midway point of the third.
Thornton's goal was his first against the Bruins in three contests since being traded to the Sharks during the 2005-06 season. It was his second game in Boston since the trade.
The Thornton marker went through a video review as the puck went off his skate and past Bruins goaltender Tim Thomas. It was, however, a clean goal as Thornton didn't kick the puck in.
The Sharks' Mike Grier added an empty-netter with 28.6 seconds remaining to cap the scoring.
Milan Lucic scored both Boston goals in the first period, while Rob Blake had the other goal for San Jose.
Lucic now has 13 goals on the season. The sophomore has already eclipsed his point total from his rookie season in 2007-08, as he now has 31 points (13 G, 18 A) in 47 contests this season. A year ago, Lucic had eight goals and 19 assists in 77 games.
The regulation loss snapped a 6-0-3 for the Bruins, who have now dropped two straight. The B's fell 4-3 in overtime to the visiting Philadelphia Flyers on Saturday, blowing a 3-1 first-period lead.
Certainly, back-to-back home losses must be a concern. In those two games, the Bruins scored five times in the first periods, only to be outscored 7-0 the rest of the way. The loss to the Sharks was Boston's first regulation defeat of the season when leading after two periods in 29 games (26-1-2).
It won't get any easier as the Bruins now embark on a five-game road trip, with their next game against the red-hot New Jersey Devils (8-2-0 in their last 10) on Friday the 13th.