19 - Number of times that the Caps have been on either side of a 3-1 record in a best-of-seven series. Eight of those 19 series have been 3-1 deficits for the Caps; they've come back to win the series twice, first back in 1988 when they downed the Flyers and more recently in 2009, when they beat the Rangers.
On the flip side, the Penguins are 10-3 all time when taking a 3-1 series lead. An impressive record, sure, but two of those three losses have come in the last five years, falling to the Lightning in 2011 and the Rangers just two seasons ago.
In fact, a 3-1 series lead really isn't what it used to be. Teams trailing 3-1 in a series have come back just 28 times in NHL history since the seven-game format was introduced, but eight of those - or roughly 29% - have taken place in the last seven years. With the exception of 2012, it's happened once in every postseason since 2009 (and twice to the Caps, as we all remember well)... and both the 2013 Blackhawks and 2014 Kings had to overcome a 3-1 deficit en route to winning the Cup.
In other words, it ain't over until someone wins four.