Hells Horses. Was System Shock 2 also the first game with mantling? I forgot it had it. I start up the game and of course the first thing I do is turn around and jump over the railing onto the tracks. Figure I'm stuck down there and I was ok with that. But then I thought I remember that if you could jump close enough to the top of something you could mantle over it. Sure enough I was able to stand directly on the track to get slightly higher and I was high enough to get the character to mantle back over the railing. So much innovation in this game.
Pretty sure Call of Duty: Ghosts is the first game to let you do that