I was playing an Indoor game, Phoenix at Orlando, Arcade mode using my Gravis, default ruleset when you cycle to indoor.
Phoenix fumbled, then the screen went black and I got the purple text saying something about object fumblemod mode something. I tried to take a screencap but it didnt capture it. It gave me the option to hit F9 and the game continued on with Phoenix still in control of the ball.
It looked like Orlando recovered, but when I came back to the game screen after hitting F9, Phoenix had the ball. Which doesnt matter to me, i've seen the ball change hands a couple times on fumbles before, but the error screen was a first. This was the first time i've encountered an error screen during gameplay. In the past it was usually in the team creation or league menus.
Sorry i cant remember exactly what it said.
I played two full games, one outdoor US and the above Indoor game. That was my only experience with it. I dont remember a fumble in the US game.