Los Blancos fell behind in the first half to a rare goal from Adrien Rabiot but Toni Kroos was brought down in the box just before the break giving the La Liga giants a chance to get back in the tie.
Up stepped Ronaldo who fired past Alphonse Areola for his 10th Champions League goal of the season.
Another goal from the Portugal international along with a strike from Marcelo meant it was advantage Real with a 3-1 win heading into the second leg in France.
But it was Ronaldo’s penalty in the first half that garnered plenty of interest on social media, and not for the foul on Kroos.
Fans took to Twitter after noticing something odd about the spot-kick.
In a slow-motion replay the ball appears to pop up just before Ronaldo strikes the ball, leading to many suggesting he illegally touched the ball with his left foot first.
Michael Owen was one of the first to notice the bizarre sequence.
He tweeted: “I reckon that penalty was a double touch and should have been disallowed #Ronaldo.”
But upon further look Owen came up with the conclusion there was in fact no touch from Ronaldo.
He added: “Or maybe the turf flicked the ball up. Checking the replays now but the ball definitely popped up.”
“It was the turf. Great volleyed penalty from Ronaldo!!!”