Gianluigi Donnarumma passed the ball straight to Real Madrid forward Vinicius Junior, which led to a goal from French striker Karim Benzema.
Donnarumma showed no urgency when receiving the ball inside his own box. Benzema applied the pressure to him, which forced the error.
Donnarumma made a fantastic save in the first-half to deny Benzema, but this mistake certainly evens it out. The goalkeeper, who is only 23-years-old, possibly showed a lack of experience, especially with this game being so significant.
The goal meant Benzema scored his 6th Champions League goal of the season and his 28th in all competitions.