The Argentine had an awful attempt at a Panenka penalty saved by the Chelsea goalkeeper as his tame effort fell into the Senegal stopper's hands in City's 2-1 defeat at the Etihad.
It proved to be incredibly costly, too, as Marcos Alonso scored a dramatic injury time winner to seal Chelsea all three points and delay City's title celebrations.
Aguero's tame spot kick came moments after Raheem Sterling gave the hosts the lead.
After Gabriel Jesus shrugged off the Dane, he squared for Aguero, who hesitated after a poor first touch before Sterling arrived to tuck home.
No one could have predicted what would happen next, however, with Mendy simply staying on his feet to make a comfortable save from 12 yards after Jesus was brought down by Billy Gilmour in the box.
You can look at Aguero's terrible Panenka, below…
On commentary duties for talkSPORT, Sam Matterface and Trevor Sinclair couldn't quit believe what they saw from Aguero.
"He's chipped the ball straight into the arms of Mendy," said Matterface.
"It's the most casual, pathetic penalty you have ever seen!"
"Yeah, the goalkeeper has read it," former City star Sinclair replied. "It was fantastic play from Mendy, he stayed on his feet and kept his nerve.
"Aguero tried to Panenka it right down the middle of the goal and it's an easy take. It's let Chelsea off the hook."
"It was just ridiculously nonchalant," added Matterface. "It wasn't even a good Panenka, it was dying before it got to the line.
"It was pretty poor, a really odd decision to make for Man City's record goalscorer with the opportunity to put the Premier League title race to bed."