At 2:17 of the first period in the Maple Leafs/Bruins game, Toronto requested a Coach's Challenge to review whether Boston was off-side prior to David Pastrnak's goal.
After reviewing all available replays and consulting with the linesman, NHL Hockey Operations staff was inconclusive in determining whether Boston was off-side prior to the goal. According to Rule 78.7, "If a review is not conclusive and/or there is any doubt whatsoever as to whether the call on the ice was correct, the On-Ice Official(s) will be instructed to confirm their original call."
Therefore the original call stands - good goal Boston Bruins.
Since the Coach's Challenge did not result in the original call being overturned, the Toronto Maple Leafs forfeit their time-out.