It's only happened in hockey history twice - and three times in the history of American pro sports. Coming back from a 3-0 deficit in a best-of-7 series happens about as often as [insert obscure reference here].
It happened in 1942, when the Toronto Maple Leafs turned the trick against Detroit. It happened in 2004, when the Red Sox (back when they were the good guys) stunned a dying New York Yankees dynasty in the ALCS. The last time it happened was in 1975, when the Pittsburgh Penguins blew a 3-0 series lead against the upstart New York Islanders. Interestingly enough, the Islanders nearly pulled it off again in the next round, forcing a 7th game but losing to the eventual Stanley Cup Champions: The Philadelphia Flyers.
While history may suggest that the odds are against the Flyers, there is still hope for the orange and black. All it takes is one bounce to go our way, to stem the tide of injuries that have prevented the Flyers from being able to skate with the slick Pens. Conventional wisdom may suggest that it's impossible to stop two superstars like Crosby and Malkin without your top 2 defensemen. But I've never been one to worry about history or conventional wisdom. Flyers in 7.
Observe the wrath of Philly: below, a Flyers fan hops in the box to rumble with legendary douchebag, Tie Domi.