There are very few plays in hockey that are more demoralizing than giving up a shorthanded goal, but the 5-on-3 shorthanded goal has got to be one of the biggest momentum-changing plays in the game, especially considering it only happens to a team about once a decade.
With a 4-2 lead, the Caps had a 5-on-3 power play for about a minute, just as the Penguins had earlier in the game. Odds are high that a team scores with a two-man advantage, so it seemed certain the Caps would be leading 5-2, and if they failed, it would still be 4-2.