At 19:58 of the third period in the Flames/Canadiens game, the Situation Room initiated a review under the terms of a Coach's Challenge to examine whether Calgary was offside prior to Sam Bennett's goal.

After reviewing all available replays and consulting with the Linesman, NHL Hockey Operations staff confirmed the Flames entered the attacking zone legally prior to the goal.
Therefore the original call stands - good goal Calgary Flames.
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NOTE: In the final minute of play in the third period and at any point in Overtime (Regular Season and Playoffs), Hockey Operations will initiate the review of any scenario that would otherwise be subject to a Coach's Challenge.