Grabovski scored the tiebreaking goal with 1:01 remaining and the Toronto Maple Leafs rallied for a 4-3 win that extended the Bruins' losing streak to three games.
Grabovski and former Bruin Phil Kessel both finished with two goals for Toronto, which scored twice in the final 6:33 against the struggling Bruins.
"Grabo was getting hit by everybody on their team and he ended up scoring a great goal -- his retribution was to score," Toronto coach Ron Wilson said.
It also was a night of retribution for Kessel, booed every time he touched the puck by Boston fans still angered over his holdout that forced a trade to the Maple Leafs in 2009. Kessel had never scored against his former team in 10 previous meetings, but ended that skid and another with a pair of goals Tuesday.
He batted a high rebound past Tim Thomas to tie the game 1-all late in the first period, then tied it again with 6:33 left with a forehand that handcuffed Thomas.
Thomas was frustrated by that goal, then furious after Grabovski's second of the night.
"I was trying to take him and the guy cutting to the back door instead of just taking the shot," Thomas said. "So I left an opening and he took it."
The Bruins took a 3-2 lead on Patrice Bergeron's 20th goal of the season with 10:52 to play, but failed to hang on and lost for the fourth time in five games.
"Unacceptable," coach Claude Julien said. "I think it's one of those situations where if you have any sense of pride, you're embarrassed about tonight."
Boston continued to struggle in its final home appearance before a six-game road trip. The Bruins were sloppy through the first two periods, then again in the final 10 minutes as Toronto came from behind and left the Bruins stunned.
The game appeared headed for overtime until Grabovski, who was dazed by Chara on a headfirst hit into the boards in the first period, took a pass from Clarke MacArthur and beat Thomas.
"I like play like that," Grabovski said. "I like when defensemen play hard against me. It gives me motivation and I play harder and faster."
Bergeron broke a 2-all tie with an even-strength goal with 10:52 left. Adam McQuaid fired a shot from the point and Bergeron was in place for the rebound despite getting smothered by defenseman Dion Phaneuf as he poked the puck into the net for his first goal in five games.
Grabovski, who drew a penalty when he was driven into the boards by Chara in the first, put Toronto up 2-1 early in the second on a lucky bounce off Boston defenseman Steve Kampfer's leg for a wide-open backhand.
The Bruins tied it at 2 with 1:26 to go in the first on a power-play goal. Reimer couldn't handle a shot from the slot by Brad Marchand and the rebound bounced up in the crease, where Campbell knocked it in. The play withstood a video review.
Kessel's first goal was also on a high rebound, but his stick was below the crossbar and the goal was deemed OK. The Maple Leafs were already on a power play when Chara was called for boarding, giving Toronto 1 second of a 5-on-3 advantage. That was enough time for the Leafs to win the faceoff and get a shot from Lupol, who hit Thomas. But the puck bounced right to Kessel for a mid-air one-timer.
Paille, coming off a four-game suspension for a blind-side hit on Dallas' Raymond Sawada on Feb. 3, put Boston up 1-0 with 11:56 left in the first when the puck deflected off a Toronto player's skate.
The Bruins announced after the game they had acquired center Chris Kelly in a trade with Ottawa for Boston's second-round pick in this year's draft. ... Phaneuf had two assists for Toronto. ... Bergeron has 20 goals in a season for the first time since 2007. ... Toronto picked up Lupul, a center, in a trade with Anaheim last week. ... Toronto was 2 for 3 on the power play. Boston finished 1 for 4.