The Arsenal goalkeeper's blunder handed Nico O'Reilly the first of his two goals in City's Carabao Cup final win on Sunday.
Hart said on BBC Radio afterwards: "It's just a bad moment. At this level the the stakes are so high and as dominant as Manchester City were, Arsenal had looked quite comfortable in those defensive situations and they've proven all season that they can ride storms.
"But they weren't able to ride this storm. The ball slipping through Kepa's fingers and O'Reilly being on hand, it is a massive moment. There's no getting away from it. And you know Kepa will be disappointed with that. But very similar to James Trafford he fully deserved his place. He's been excellent as a cup goalkeeper.
"He was he was pretty much faultless throughout the game. But you know what it's like as a goalkeeper that big moment. I think electing to try and catch a ball that was rising was tricky. He didn't quite do it.
"I'll always say it. Don't ever feel sorry for a goalkeeper. That's the the life that he's chosen. Again, most of the game he was faultless. But in that big moment he'll have something to reflect on."