I recently downloaded for FSX the Convair B-36H after watching the movie "Strategic Air Command". On my first attempt at a takeoff I was unable to get the beast off the ground before running out of runway, and I had picked a field with a long runway. When next I tried I discovered that the person who had designed the package had overloaded the aircraft to the tune of 100,000 lbs of fuel pushing the weight way above max gross. After changing the fuel load to keep the plane below max gross weight it took off well before the end of the same runway I had used before.
OK, maybe I should have thought to check the weight and balance before attempting a takeoff but some of us aren't as serious about doing preflight checks as others are. I use FSX for enjoyment and not to simulate in every detail an actual flight. And this wasn't the first aircraft that I have found to be over weight as is.