Canadiens goaltender Carey Price appeared to have frozen the puck after stopping a backhander by James Neal, but Neal poked it from under his glove and Letang rapped it into the net in for the game-winner.
Price argued he clearly had his hand over the puck before it was dislodged, but no avail, and the Atlantic Division-leading Penguins improved to 14-6-4.
Marc-Andre Fleury stopped 24-of-27 shots to record the win.
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