The Flyers' 1979-1980 year should have been remembered for its triumphs--a 116-point regular season, the highest in the league and the best (even to date) in franchise history, including an NHL-record 35-game undefeated streak, capped with a trip to the Stanely Cup finals against the New York Islanders. Instead, it is primarily remembered for one man, who donned the Black and White rather than the Orange and Black: Leon Stickle, whose no-call in Game Six of the Finals on May 24th, 1980 helped the Islanders seal the game for the series win.
Stickle's brush with infamy came late in the game's first period, when Islanders winger Clark Gillies appeared to send a pass back over the blue line for teammate Butch Goring, who proceeded to advance into the offensive zone. The play looked to be offsides, and the Flyers defensemen reacted as such, but Stickle ruled a safe pass, and Goring's feed to a streaking Brian Sutter led to the Islanders scoring a relatively easy goal. Stickle would later admit that he blew the call, securing his place in Philadelphia immortality for the most undesired of reasons.
Though it's true that the Flyers ended up losing the game, one thing should be said in Stickle's defense: The goal wasn't really as consequential as you might think. When you hear people rant about how a no-call cost the Flyers the game and the series, you figure it was Bob Nystrom's famous overtime goal that should have been nulled, or that at least it was the game-tier late in regulation. In actuality, Sutter's goal merely put the Islanders up 2-1 with two whole periods to go, in what would eventually end up a 5-4 game. Sure, that's still a one-goal differential, and who knows how things would have turned out with the violation called, but as far as all-time scapegoats go, this isn't really Buckner letting the ball through his legs in the bottom of the tenth.
Still, was the call definitely off-sides? Well, you judge for yourself. But yes. Yes it was.