At 19:40 of the second period in the Senators/Capitals game, Ottawa requested a Coach's Challenge to review whether Washington was off-side prior to Karl Alzner's goal.
After reviewing all available replays and consulting with the linesmen, NHL Hockey Operations staff confirmed that Washington's Justin Williams had possession and control of the puck as he entered the attacking zone prior to the goal. According to Rule 83.1, "a player actually controlling the puck who shall cross the line ahead of the puck shall not be considered 'off-side,' provided he had possession and control of the puck prior to his skates crossing the blue line."
Therefore the original call stands - good goal Washington Capitals.
Since the Coach's Challenge did not result in the original call being overturned, the Ottawa Senators forfeit their time-out.