If you're near a multiple of 90 degrees for any of the axes of a 3D joint, wierd things can happen. The trouble is that there is more than one set of three angles that can represent the same rotation. The Softimage representation of rotation with three fcurves thus has inherent problems. It's possible to get Softimage itself into trouble this way, when trying to represent tumbling motion. This may get better with Softimage Sumatra or with later versions of Falling Bodies .
Gimbal lock occurs when the first and third axis of a joint line up.
The simulation will be very slow if a completely straight limb hits an obstacle hard. In the real world, bones break when this happens. Falling Bodies struggles to find a solution that makes sense in such situations, but it takes minutes. You can prevent this by setting the joint limits so that knees and elbows can't quite make it to the straight position. One or two degrees short of straight is enough to stop this problem.